Home grown by Nation i by Club

I've started new challenge with a friend of mine and idea is to sign only players that are or will be trained by club - it is hard as there is no info how is that calculated - the idea is playars has to be in club for 3 years before 21...but i just wanted to ask more info if anyone has - cause in one point you do sign player of 18 and he is not at the end train by club, but in other cases he is...
So please if anyone has info about it i would apreciate very much..
Tnx guys