There have been a few threads in the past that give hints on what the formula looks like. There are a number of factors:
- potential
- work ethic
- playing time
- starts
- the starting value of the attribute
- practice time
- number of games
- other factors that provide some variability in each result
It is not a straight line progression - it can be approximated that way for ratings that start below 70 - 80, but above 80, you definitely see a curve as the growth diminishes.
I tracked everything for all my recruits for 30 seasons and then ran some statistical simulations to come up with the ultimate formulas I use now. You can get the straight line approximation after tracking a season or two and refine from there, so it shouldn't take you long to get rolling.
Hope this helps,
B