Overview:
43 Rings have been tested
8 are fixed
7 are not fixed
28 are OK
35 are untested
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Résumé:

The fixed Rings:
- Chain Lightning
- Energy Barrage
- Explosive Shot
- Firestorm
- Flashfire
- Pommel Strike
- Walking Bomb
- War Horn

Damage faulty Rings that won't change (when ConstValue is changed). So they remain unchanged:
- Grappling Chain (Upgrade 'Give them the Boot')
- Leaping Shot

Duration faulty Rings that won't change (when ConstValue is changed). So they remain unchanged:
- Cloak of Shadow
- Counterstrike
- Fire Mine
- Rally
- Rampage
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Details
The following list shows the numbers as they were, then the proper modded values.

Descriptions:
Increased Ability damage by 30%
Increased Ability duration by 30%.

1) The fixed ones

Chain Lightning  (ChainLightning)
Displayed in the Abilities screen: Electric damage : 300% Weapon damage (250% without the ring).
An increase of 50. That is 20% of 250!
The proper number is now: Electric damage : 325% Weapon damage (250*0.3=75 and 250+75=325).
---
Energy Barrage  (SeekerSwarm)
Displayed in the Abilities screen: Electric damage : 78% Weapon damage (66% without the ring).
An increase of 12. That is 18.18% of 66!
The proper number is now: Electric damage : 86% Weapon damage (66*0.3=19.8 and 66+19.8=85.8).
---
Explosive Shot  (ExplosiveShot)
Displayed in the Abilities screen: Damage : 230% Weapon damage (200% without the ring).
An increase of 30. That is 15% of 200!
The proper number is now: Damage: 260% Weapon damage (200*0.3=60 and 200+60=260).
---
Firestorm  (ChargedBurstFire)
Displayed in the Abilities screen: Fire Damage : 180% Weapon damage (150% without the ring).
An increase of 30. That is 20% of 150!
The proper number is now: Fire Damage: 195% Weapon damage (150*0.3=45 and 150+45=195).
---
Flashfire  (FlameTongue)
Displayed in the Abilities screen: Fire Damage : 435% Weapon damage (300% without the ring).
An increase of 135. That is 45% of 300!
The proper number is now: Fire Damage: 390% Weapon damage (300*0.3=90 and 300+90=390).
---
Pommel Strike  (PommelStrike)
Displayed in the Abilities screen: Damage : 366% Weapon damage (300% without the ring).
An increase of 66. That is 22% of 300!
The proper number is now: Damage: 390% Weapon damage (300*0.3=90 and 300+90=390).
---
Walking Bomb  (WalkingBomb)
Displayed in the Abilities screen: Spirit damage: 283.29999% Weapon damage (200% without the ring).
Explosion damage: 583.29999% Weapon damage (500% without the ring).
An increase of 83.3 for both. That is 41.65% of 200 and 16.66% of 500.
The proper number is now: 0.8 instead of 0.833. It is an overall damage increase of 35% (because it needs some tending).
Correct Damage % over 10sec: 700+175=875
Single value for both damages: 800+80=880  (so, roughly the same).
---
War Horn  (WarHorn)
Displayed in the Abilities screen: Duration : 7.5 sec (6 sec without the ring).
An increase of 1.5. That is 25% of 6!
The proper number is now: Duration : 7.8 sec (6*0.3=1.8 and 6+1.8=7.8).
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2) Damage faulty Rings that won't change (when ConstValue is changed). So they remain unchanged:

Grappling Chain  (GetOverHere) (the upgrade 'Give them the Boot').
Displayed in the Abilities screen: Damage : 200% Weapon damage (200% without the ring).
An increase of 0. That is 0% of the infinite!
The proper number should be: Damage: 260% Weapon damage (200*0.3=60 and 200+60=260).
---
Leaping Shot  (JumpShot)
Displayed in the Abilities screen: Damage : 50% Weapon damage (50% without the ring).
An increase of 0. That is 0% of what have you.
The proper number should be: Damage: 65% Weapon damage (50*0.3=15 and 50+15=65).
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3) Duration faulty Rings that won't change (when ConstValue is changed). So they remain unchanged:

Cloak of Shadow  (?)
Displayed in the Abilities screen: Duration : 3/6/9 sec (3/6/9 sec without the ring).
An increase of 0. That is 0% of any shadow!
The proper number should be: Duration : 4/8/12 sec
---
Counterstrike  (Retaliation)
Displayed in the Abilities screen: Duration : 6/12/20 sec (6/12/20 sec without the ring).
An increase of 0. That is 0% of nothing!
The proper number should be: Duration : 7.8/15.6/26 sec
---
Fire Mine (FireMine)
Displayed in the Abilities screen: Duration : 3 sec (3 sec without the ring).
An increase of 0. That is 0% of what have you!
The proper number should be: Duration : 2 sec (3*0.3=1 and 3-1=2).
---
Rally  (CallOfTheAncients)
Displayed in the Abilities screen: Duration : 15 sec (15 sec without the ring).
An increase of 0. That is 0% of anything!
The proper number should be: Duration : 19.5 sec (15*0.3=4.5 and 15+4.5=19.5).
---
Rampage  (Ammo)
Displayed in the Abilities screen: Duration : 10 sec (10 sec without the ring).
An increase of 0. That is 0% of happiness!
The proper number should be: Duration : 13 sec (10*0.3=3 and 10+3=13, yep).

All four Focus based Abilities whose duration is supposed to be increased show no change. Maybe those rings are nonetheless effective in-game, but I wouldn't trust that. It is more likely they are bugged.
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Notes:
- I would be glad to learn how to make the faulty unchanged Rings display the proper numbers. The Posts tab is open for suggestions.
- All the above is about what is displayed in the Abilities screen. How the numbers are treated in-game is something I haven't tested.
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Here are the 28 Ability Rings with correct values. The percentages vary between 30 and 35.7%:
Barrier, Blizzard, Block and Slash, Caltrops, Challenge, Charging Bull, Deathblow, Earthshaking Strike, Horn of Valor, Ice Mine, Immolate, Knockout Powder, Long Shot, Lunge and Slash, Mighty Blow, Parry, Payback Strike, Shadow Strike, Spike Trap, Spinning Blades, Thousands Cuts, Throwing Blades, Toxic Cloud, Twin Fangs, Wall of Ice, War Cry, Whirlwind, Winter's Grasp.
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And here are the 35 Ability Rings I haven't tested:
Blessed Blades / Bodyguard / Devour / Dispel / Disruption Field / Dragon-Rage / Elemental Mines / Fallback Plan / Flank Attack / The 3 Flasks... / Full Draw / Hail of Arrows / Hidden Blades / Hook and Tackle / Knockout Bomb / Lightning Bolt / Line in the Sand / Mark of Death / Mind Blast / Poisoned Weapons / Pull of the Abyss / Resurgence / Revival / Ring of Pain / Spell Purge / Spirit Blade / Static Cage / Stealth / Stonefist / To the Death / Walking Fortress / Wall of Fire / Wrath of Heaven.

Anyone is welcome to test them and report here those rings whose stats are erroneous.
With a game editor, Abilities are in DA3\Party\Mage...Rogue...Warrior.

Important note: Beware Bioware! Testing the Abilities stats in Haven/Skyhold/Winter Palace only works for the main character. The other characters' Abilities stats are not their own there!
Elsewhere, if you want to see the correct Abilities stats of a character, you need to enter the Abilities screen while controlling that particular character in the 3-D screen! Yep, don't blame the messenger.
See the Tab 'Articles' about that trust breaking find, for details.
The problem in the HQ is that you can't control the other characters... And if we dig a bit more, the problem is Bioware.
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Note: I considered the possibility Bioware changed the numbers for re-balancing purposes but I find the arguments unconvincing.
If 30% is too much for the upgrade, that inevitably means the base damage is also too high. If re-balancing is in order, they should start from the beginning.
If they did something of the sort, it has been casual, as so many other things in this interface.
So, 30% is 30% and is good enough in practice.

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Installation
Can be installed/uninstalled anytime.
The mod has been made with the Frosty Editor. You can download the mod manager there: Frosty Mod Manager and Editor
For those who don't know yet, the Frosty Mod Manager converts the .daimod files into a format it can manage so that simplifies things to use exclusively that mod manager.
But not all Daimod mods are properly converted by the Frosty mod manager, though.
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This mod is complementary to these bug erasing mods:
Bugs Be Gone
Bugs SBFixes
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Credits: To svartalfimposter who, in a spirit of cooperation, kindly showed me where to apply the bandages.