I thought about this when reading the Release Notes post...
Retakes only add the number of upgrades to a 'base' starting % for the new stat:
ATK/HP % start at 6% while the rest start at 3%. So if you upgraded HP twice it would be 12% (6+two upgrades of 3%).
BUT if you reroll ATK/HP and get any other stat, it will be 9% (3+two upgrades of 3%)
I wonder if retakes can hit flat stat ATK/HP...oof that would rough...no one wants that...