This formula that I estimate is about as accurate as $18M in advance scouting. It works on most all the fielding, hitting, and pitching ratings. It's a pretty simple formula. Take the % of growth between the values of spring training S1 and spring training S2 multiply times 2.5 and add that number to original spring training S1 value.
If you want to use excel just put the rating of the first season in the A2 field and the second season in the A3 field and copy the following formula in the A4 field.
=((A3-A2)*2.5)+A2
***(edit thanks to mrpolo09 for cleaning up the formula a bit)
It has some limitations. First off it takes two spring trainings to work so it's not immediate. It won't work on players created in the initial league setup, so they need to be drafted players or IFAs. Injuries and DITR also mess with the formula. It also won't tell you when a guy starts to decline. So it's not perfect, but by testing it on vets I've found it is rarely off by more than a few points.
I first used it on my players and was getting consistent and accurate results, but then I started checking other teams and notice it worked just as well. Some of these teams were owned by completely incompetent owners and yet they still progressed.
Besides being useful for predicting players it did also seem to indicate that unless a player is just blatantly abused on purpose, had an injury, or became a DITR, there is not much that you can do to affect a players ratings. That doesn't mean that coaches don't affect players, but the truth is since coaches jump around and we all hire coaches with roughly the same ratings the net affect is very small.
It also seemed to indicate that every player pretty much uses the same development and the only things that can create a large change are injuries, DITR, and at what age a player declines.