My personal opinion: if you see growth chance for a relevant stat is at 86% or greater then level that thrall normally with food, 'cause if you bump stat growth to 100% then RNG can’t say no and then thrall MUST get at least 20 points of growth for that stat.
Usually the actual stat growth for 100% growth chance is around 30 points, can be lower, can be higher as additional points gain is still RNG that cannot be affected in any way.