No one really knows exactly how potential is worked out. However, it is a combination of skill total and salary. And since salary and skill totals are linked, you can see how potential and salary are linked.
The estimates provided are based on peoples experiences and when they notice that training has slowed significantly then they realise they have hit the cap, and they take note of the level of salary at that time.
Inside skills do add more to the salary for big men than guard skills do for guards.