Yeah, everyone assumed 70% was the requirement, but you actually just have to make specific dialogue choices during Johnny's "gravesite" scene, where you basically become (or not) friends. It's sort of like how you can befriend Takemura during the stakeout. I don't know what the point of Johnny's synchronization meter is, unless they had originally planned something for it or are planning something for it in the future.
4 comments
Please update this save game in Ultimate Edition too
It is required to unlock the secret ending