That is a good question. I don't know the answer. Most of my players are within two levels in their main skills. My starting center has a bit wider margin.
If you trainee is 18 and you want him to be a great player for your team, you should figure a training schedule that is four seasons long and stick to it. Aside from the main skills, you should give your center some one on one for forwards, some passing, and for sure free throws. Big men shoot more free throws in general (especially if you play inside tactics) and they can be the difference. My advice would be to mix those other trainings in occasionally, in place of SB.
Once I scored a basket that still makes me laugh.