Here I dont have the formula right now but keep in mind that white skills gain double XP, then theres a reduction on xp gaining that is around a 15% once you win a star(*) X number of skills that the drill affect.
Time ago you it was linked to the main star so once you reached 80, 100, 120... The gaining had a reduction but now for ex. If a drill affect x2 skills at 120, -so 60 x2- 160 so80 x2... 200, 240 etc theres a reduction on xp gaining. Remember that every player gains xp differently based on age slab and too if he's well programmed as fast trainer. Thats why you have to test players once you sign them, train a role or SA and check the gaining so to know if hes fast trainer and too, test the team vs a 1"% team so this align all your players at the 9* and you can compare a new signing, 4* 5* etc VS your best AvQ players by doing that.