Also, if you end up needing to write them a check at the end of the year, there are two ways to calculate your I-didn't-withhold-enough penalty: you can assume you made your money at a constant rate through the year, or you can do the more difficult thing and calculate what you really owed them on a quarter-by-quarter basis, and only pay a penalty if you were behind. If you do the latter, you don't actually need to predict very much.