I uaw 100% main force in Windows (with damper/springs set to 0), and a varying amount in game.
Normally around 40% for low power FWD's, 20-25% for RWD's, and 40-50% for single seaters. Works good for me and provides a good amount of useful feedback without being over the top or overpowering.
I have LFS/Windows set to 720 degree rotation, LFS automatically adjusts for cars with less rotation.
Remeber you can use the < and > keys to adjust FF strength ingame.