Trial and error. First off, you have to back up the league or everything becomes quite tedious. Then, I enter numbers and if I like the results, I move on or I restart with different numbers. The key is to get results that YOU will be happy with. I do not use the recalculate button or IatricSB's ERA calculator, although I have tried both. Recently I have gone back to using Lee's spreadsheets. This is an excellent starting point for league totals. From there...TWEAK, TWEAK and TWEAK. Right now I am working on park effects to see how much of an influence they have on my results. I have other league totals up until the '60's but these were used with OOTP3 and they may have to be adjusted.
Here is a link for Lee's spreadsheets:
<a href="http://www.sportplanet.com/sbb/files/adjustedtotals.htm" target="_blank">http://www.sportplanet.com/sbb/files/adjustedtotals.htm</a>