Let’s do some math, shall we?
For this example we’ll use a volcano fighter, which starts with 15/3/10/2/15.
At level 10, your only choice of perks (due to level restrictions) are the “Level 0-10” set (+1/2/3 to all stats) which are good, the “Level 0-15” set (+3/+3 to stats, should probably include Thick Skull but doesn’t and shouldn’t include Relentless but does) which are good, the “None” set (+3 to a stat) which are good, and the “STAT 9-99” set (-3/+5) which could be good if you got -Acc or could be bad if you got -Str.
Note that in the 9-99 set:
- You are eligible for the -Str and -Surv perks at spawn
- There are 2 -Str perks and 2 -Agi perks, but only 1+Str reward
- Even at a 50% chance to increase you will likely end up with 9 or higher Agility because 50% of 10 is 5, and 40% of 5 is 2, so you should average 7 points and plus the 3 you get at spawn you will likely have 10 or higher Agility without food buffing it
- You might be in the range for the negative Accuracy perk, but since you only get 2 at spawn and assuming the same averages that would only put you at 9 so you might also miss it.
- Of this set, 4 are a bad outcome since you lose Agility or Strength, while only 1 is a good outcome.
Chances are tho, at 10 you will get a decent perk, since the desirable outcomes outweigh the undesirable outcomes by quite a bit.
At level 15, you have lost the “Level 0-10” set and have picked up a second set of “9-99” perks with a -5/+8 chance, and while there are 2 -Acc perks there is now a -5 Vit perk, which is not something you want on any thrall. So you’ve lost the ability to gain 3 good perks but picked up the chance for 3 bad ones and 2 good. By level 15 you will almost definitely have a 9 in each stat, and in no way could you be over the 99.
You really aren’t eligible for the “STAT 3/5-10” set, since you lost the ability for +5 Survival, Strength, and Vitality at spawn (by being at 10 or higher) and by 15 you would have gained an average of 10.5 to every stat (15 x 50% = 7.5, 7.5 x 40% = 3), so unless you rolled a -5 to Accuracy, you would always be well over the 10 point barrier regardless of what you boosted with food, even if you were at a 50% chance of growth. So the only possible perk you could even get from this set is +5 Accuracy, which doesn’t hurt but isn’t really a good outcome either.
Notice so far that the % increase of any stat has not at all affected what you will get (we’re assuming 50% in any stat with no food boost) except possibly Accuracy so I guess you would recommend boosting Accuracy via food on a fighter?
At level 20 you lost the “Level 0-15” set, which were all good perks, in order to pick up the “11-99” set, which you will always be in range of because you are gaining an average of 14 points in any given stat so you should be well over the 11 but well below the 99 cap.
Still failing to see how the % chance for increase, boosted or not, has any effect since you very rarely get anything under 50%, but if you want to believe that it does, feel free.
I didn’t mention the perks that give you +15 Vitality, since a volcano fighter has a base health of 4950 so isn’t eligible for the first 2 and might roll the third but it’s pretty rare, given the sheer number of options you still have at level 20.
TLDR: Your chance to get a bad perk increases at higher perk levels, while your chances at a good one are pretty much cut in half, and the % chance to increase don’t make a difference in the perks you get, food boosted or otherwise.