Thanks Der_Ryan_M.

I guess the effectiveness (how many +1 by how many drills) have to do with multiple factors right ?
e.g.
- number of whistles (1,2 or 3)
- potential of player
- age maybe

I applied 15 (5x3) drills to an AMC who assigned him an AMR (22 yo - 100% 30 ability) and he got just 1/50 . Is it normal?