If you don't have a particular game where you're comfortable with your aim that you want to convert from, the best way in MW2 is to use relative ADS sensitivity and start from 0. This is the slowest "regular" conversion without adjusting the individual scopes. If this feels too slow, start increasing the coefficient gradually until it feels more natural. The typical range used is 0-178, but any value is valid as long as it works for you.
When you find your magic number you can configure the calculator to match this (using Monitor Distance - Vertical and the coefficient) to easily convert this to other games.