I'm playing on PS3 with the DFGT, I assume the settings will be similar.
I set up my steering saturation to about 50% and linearity to 25%. The numbers vary slightly according to which car I use.
You'l have to note that when you're turning the wheel too far, there's suddenly no more FFB. Kind of hard to explain, but you'll know what I mean when you experience it. It feels like you've literally turned your wheel too far (ingame, that is).
Don't go beyond that point. Your steering will not increase, and it will only increase the amount of steering you will have to do when you turn it back upright. Also, in the Marussia for some reason, going past this point means you will be using less steering. Can't explain why.