"Average drill quality" is the average of skills that the drill give

for example skill drill trains creativity dribbling and heading
so if a player has 109 creativity, 113 dribbling and 90 heading the average would be 104

now keep in mind that the maximum average a drill can have is 180, beyond that point your player would no longer gain experience training
so if for your player all the above 3 skills are white, and we know the average is 104, we can say that we are left with 180-104% to train = 76% meaning that if you use skill drills as many times as you can until you reach the maximum each of the 3 drills would each increase by 76% resulting in 185 creativity, 189 dribbling and 166 heading for a total average of 180

here the sheet asks you how far you want to use a drill, do you want to use it until the average of the skills is 140? or do you want to use it to the max? all the way to 180?

the higher the value you set for average drill quality the more greens that would be required to train

Please note that if a drill gives both whites and grey skills, the experience is gained at a ratio of 2:1

so if our previous player was only an AMR (which would make heading grey), then we know that he would gain 2 points of creativity and 2 points of dribbling for each point of heading he would gain so in that case, if you use skill drill to 180 average you would have a player with 200 creativity, 204 dribbling and 135 heading

sorry if its not well organized