Mortster,
Here is a post I found which is very useful, the example at the top of the post is 720 rotation with 26.5 lock = 13.5:1 ratio. That is the approximate steering ratio of a BMW E46.
http://forum.racedepartment.com/gtr-evolution/13702-steering-ratios.html#post296625
I've settled on 540 degrees now in the Logitech Profiler and using the Car Setup page in the Garage have set 18 degrees on steering lock.
Made my lap times slower for awhile but now back into the zone of 1:35-39 per lap on Brands Hatch
Another thread discussing the G25 but settings apply to DFP as well -
http://forum.racedepartment.com/gtr-evolution/1137-g25-settings.html
Steering feels much more natural now, have increased FFB settings to medium as well.
My other settings in the Logitech Profiler are set as this guide below suggests for Live For Speed (which I have downloaded only had a quick blast on it the FFB was great):
http://en.lfsmanual.net/wiki/Wheels#Correctly_setting_up_a_Logitech_Wheel
WhiteKnight