It’s not quite as cut and dried as that, but that is the general rule.
It is a character by character basis too, but generally, somewhere around the g12 threshold a characters base stats will have improved to the point that the % based boosts yield higher returns.
If you have a mod with both flat offense up and percentage offense up (for the examples sake, let’s say that the flat offense hit 3 times for +100, and the percentage offense hit twice for 2%).
If you have a g11 attacker with 4000 base offense that equals +100 for the flat and +80 for the 2% for 4180 total offense.
You increase them to g12 (and here it is important to remember that individual gear pieces are added to the character base as a bonus after the the mod bonuses are applied, but when you increase to the next gear level All those bonuses are applied to the base stats), and now you have 5000 base offense, now both the flat and the % bonus add 100 and are equally beneficial.
Increase to g13 and 6000 base offense, now the % boost is 120, and the flat is still 100, percentage based mod is yielding better returns.
At relic 7, with 7500 base offense, now it is up to 150 offense For the % boost, while the flat increase is still 100.
Hope that helps.
