I reckon about 400 Greens to get him to 120%. Double that to 8/9* but others might be able to give a more accurate figure.
I am cooking a GK in Youth Academy at the moment. He is on 120% already so I plan to get him up to at least 8* and take it from there. Only because I cannot be arsed to power train a 3*. Yes the Greys will be higher but it’s a GK, not an AML/AMR.