I use a non-scientific formula that is pretty accurate at predicting batting average and on base percentage for hitters and BAA and OBPA for pitchers. However, it is less accurate at predicting slugging. I use it and I seem to be doing okay. Some say they have better more scientific formulas that seem to work for them.
You can grab twenty similar players, look at their stats and back yourself into a formula. Then do it again with twenty more similar players, then rinse and repeat a few times and you will likely come up with something yourself.