We currently don't (and can't) know.
If you want a way to convert from other games, mousesensitivity.com works really well and even supported Sandstorm during Beta 1. Then 2 rolled around and messed up all their parameters so they've taken it down temporarily. You will have to pay for an account for the more complex conversions though. Well worth the price of lifetime membership if you play a lot.
Probably not the answer you want, but with no way to make a conversion now, I use the measuring method. I measure on my mouse pad how many centimetres it takes to do a 360 in Insurgency Source and then change sensitivity in Sandstorm to match those cm.. It's not the most accurate way, but I use for all my FPS games and is working fine.
Probably the most difficult part is to match the scoped sensitivity. Didn't change anything there, but probably will, once the overall aiming aspect of the game becames less "draggy" and more snappier.