Geralt in No Levels has less health compared to his enemies, than in original game, so there is less damage to be returned. And Geralt has less armor points compared to incoming damage, than in original game. This made some already bad "return damage" things even worse. So I did some math, to make them at least as effective as in original game, on medium difficulty (2/4), and on average level these things can be used throughout the game.

Data for calculations {
Highest quest level in the game: 49. Rounded up to 50.
Geralt health at levels:
Lvl 5 = 3900
Lvl 10 = 4300
Lvl 15 = 4600
Lvl 20 = 4900
Lvl 27 = 5250
Lvl 30 = 5380
Lvl 32 = 5440
Lvl 35 = 5530
Lvl 42 = 5725
Lvl 50 = 5925

Level 20 drowner essence
    Medium difficulty 2123 <health_final_multiplier type="add" min="0.1" />
    Hard difficulty 2316 <health_final_multiplier type="add" min="0.2" />
    (Base drowner health 220 + 19 * 90 level bonus) * difficulty 1.1 or 1.2 = 2123..2316
}
Black blood potion. {
    Level 1 recipe starts dropping from containers on level 5. Returns 15% damage.
        Best scenario. Level 5 drowner has (220 + 4 * 90) * 1.1 = 638 HP. Geralt with his 3900 HP can kill with returned damage 91.7% of a drowner.
        Medium scenario (before upgrade). Level 10 drowner has (220 + 9 * 90) * 1.1 = 1133 HP. Geralt with his 4300 HP can kill with returned damage 56.9% of a drowner.
        Whole game average scenario. Level 27 drowner has (220 + 26 * 90) * 1.1 = 2816 HP. Geralt with his 5250 can kill with returned damage 28.0% of a drowner.
        Worst scenario. Level 50 drowner has (220 + 49 * 90) * 1.1 = 5093 HP. Geralt with his 5925 HP can kill with returned damage 17.45% of a drowner.
    Level 2 recipe starts dropping from containers on level 10. Returns 20% damage.
        Best scenario. Level 10 drowner has (220 + 9 * 90) * 1.1 = 1133 HP. Geralt with his 4300 HP can kill with returned damage 75.9% of a drowner.
        Medium scenario (before upgrade). Level 15 drowner has (220 + 14 * 90) * 1.1 = 1628 HP. Geralt with his 4600 HP can kill with returned damage 56.5% of a drowner.
        Whole game average scenario. Level 30 drowner has (220 + 29 * 90) * 1.1 = 3113 HP. Geralt with his 5380 HP can kill with returned damage 34.6% of a drowner.
        Worst scenario. Level 50 drowner has (220 + 49 * 90) * 1.1 = 5093 HP. Geralt with his 5925 HP can kill with returned damage 23.3% of a drowner.
    Level 3 recipe starts dropping from containers on level 15. Returns 30% damage.
        Best scenario. Level 15 drowner has (220 + 14 * 90) * 1.1 = 1628 HP. Geralt with his 4600 HP can kill with returned damage 84.8% of a drowner.
        Whole game average scenario. Level 32 drowner has (220 + 31 * 90) * 1.1 = 3311 HP. Geralt with his 5440 HP can kill with returned damage 49.2% of a drowner.
        Worst scenario. Level 50 drowner has (220 + 49 * 90) * 1.1 = 5093 HP. Geralt with his 5925 HP can kill with returned damage 34.9% of a drowner.
    Can be used only against necrophages and vampires.
}
Leshen decoction {
    Easiest leshen is level 20. Returns 10 and 10% damage.
        Humans.
            Best scenario. Level 20 human has (25 + 19 * 55) * 1.1 = 1177 HP. Geralt with his 4900 HP can kill with returned damage 490/1177 41.6% of a human. 580/1177 29.2% if we add 90 damage from about 9 attacks which human needs to kill Geralt.
            Whole game average scenario. Level 35 human has (25 + 34 * 55) * 1.1 = 2084.5 HP. Geralt with his 5530 HP can kill with returned damage 26.5% of a human.
            Worst scenario. Level 50 human has (25 + 49 * 55) * 1.1 = 2992 HP. Geralt with his 5925 HP can kill with returned damage 19.8% of a human.
        Drowners.
            Best scenario. Level 20 drowner has (220 + 19 * 90) * 1.1 = 2123 HP. Geralt with his 4900 HP can kill with returned damage 23.1% of a drowner.
            Whole game average scenario. Level 35 drowner has (220 + 34 * 90) * 1.1 = 3608 HP. Geralt with his 5530 HP can kill with returned damage of 15.3% a drowner.
            Worst scenario. Level 50 drowner has (220 + 49 * 90) * 1.1 = 5093 HP. Geralt with his 5925 HP can kill with returned damage 11.6% of a drowner.
        Whole game average (26.5% + 15.3%) / 2 = 20.9%
    }
Retribution, level 3 glyphword. {
    Available after level 32 quest.
    Search for 'Glyphword 5 _Stats' in code.
    Always returns the same damage = armor points * 0.67
        Medium difficulty. {
            Early scenario. Level 32. {
                Player armor: 36 + 32 * 11 = 388
                Human attack damage: (200 + 100 + 31 * 20) * 1.4 = 1288
                    NPC Short sword 1 _Stats + ConDefault/light_attack_damage_vitality + 31 * NPCLevelBonus/light_attack_damage_vitality
                Human attack vs armor: 388 * 0.3 * 0.67 / 1288 = 6.0% damage returned
                    0.3 * 0.67 = 0.201
                Drowner attack damage: (250 + 0 + 31 * 5) * 1.4 = 567
                    Drowner does not light_attack_damage_vitality, instead it has attackAction.powerMod = {valueAdditive=0, valueMultiplicative=1.2, valueBase=95}. 95 is level 20 is 19 * 5 = 95. valueMultuplicative was 1.2 from WeatherBonusEffect.
                Drowner attack vs armor: 388 * 0.2 / 567 = 13.7% damage returned
                Even less returned damage for big monsters. Because returned damage depends on number of received attacks which deal damage to Vitality, and amount of armor points.
                }
            Whole game average scenario. Level 41. {
                Player armor: 36 + 41 * 11 = 487
                Human attack damage: (200 + 100 + 40 * 20) * 1.4 = 1540
                Human attack vs armor: 487 * 0.201 / 1540 = 6.35% damage returned
                Drowner attack damage: (250 + 0 + 40 * 5) * 1.4 = 630
                Drowner attack vs armor: 487 * 0.2 / 630 = 15.4% damage returned
            }
            Endgame scenario. Level 50. {
                Player armor: 36 + 50 * 11 = 586
                Human attack damage: (200 + 100 + 49 * 20) * 1.4 = 1792
                Human attack vs armor: 586 * 0.2 / 1792 = 6.5% damage returned
                Drowner attack damage: (250 + 0 + 49 * 5) * 1.4 = 693
                Drowner attack vs armor: 586 * 0.2 / 692 = 16.9% damage returned
            }
        }
    I will go with (6.35% + 15.4%) / 2 = 10.875% whole game average.
    With No Levels, current average armor has 45 points. Human sword damage 200.
    45 * 0.3 * 0.67 / 200 = 4.5% damage returned.
    To achieve 10.875% I need
    0.10875 * 200 / 0.3 / 45 = 1.611
}
Toxic Blood mutation. {
    Returns 3% of damage per 1 toxicity point.
    I will use 300% returned damage with 100 toxicity.
    Available after level 35 quest.
        Humans.
            Best scenario. Level 35 human (25 + 34 * 55) * 1.1 = 2084.5 HP. Geralt 5530 HP. 7.96 humans killed.
            Whole game average scenario. Level 42 human (25 + 41 * 55) * 1.1 = 2508. Geralt 5725 HP. 6.85 humans killed.
            Worst scenario. Level 50 human (25 + 49 * 55) * 1.1 = 2992 HP. Geralt 5925 HP. 5.94 humans killed.
        Drowners.
            Best scenario. Level 35 drowner (220 + 34 * 90) * 1.1 = 3608 HP. Geralt 5530 HP. 4.6 drowners killed.
            Whole game average scenario. Level 42 drowner (220 + 41 * 90) * 1.1 = 4301 HP. Geralt 5725 HP. 3.99 drowners killed.
            Worst scenario. Level 50 drowner (220 + 49 * 90) * 1.1 = 5093 HP. Geralt 5925 HP. 3.49 drowners killed.
        Whole game average (6.85 + 3.99) / 2 = 5.42 killed.
        To do that with 1000 HP and 100 toxicity, need 542% returned damage. That is 5.42% returned damage per 1 toxicity point.
        Seems good enough with 300%, I will probably leave it as it is.
}