i have 2 Perrenial allstar^potential trainees both are listed as PF.
But one is being trained to become my starting C and i'll keep the other as PF.
Respectively the C trrainee has 8 OD the PF has 9 OD.
Wich IMO is plenty for their potential.
Everything depends on the lvl of players that you have on your teams and the teams you face, but i'd say try to get at least 5 in the lower leagues.