The FF output from the game should, if it's done properly, act as a centering spring - If you want to take full advantage of the FF system in a given game, you should have it set to 0%. You
can have it on to suppliment the FF, but I'm personally against the idea as I prefer the idea of getting exactly what the game is telling me.
Spring effect strength and damper effect strength should be set to 0% - they're more there for compatability with old games than anything else. Your global effects strength though should be set to 10
1% due to a tiny glitch in the drivers; you'll get slightly more nuanced FF that way.
As for steering wheel rotation, I've done a bit of research on this recently and come up with a few things:
(2 x [Max] Steering Lock) x Steer Ratio = Rotation
Rotation/(2 x Steer Ratio) = Steering Lock
Ratio = Rotation/(2 x Steering Lock)
If you want to emulate the car you're driving 1:1, you need to know it's steering ratio. A road going BMW E46 has a ratio of 15.4:1 (3 turns), I happen to know, so:
1080/(2 x 19.6) = 35.0 ideal lock, if we had a 1080 degree wheel.
'Wrapping' this to a maximum allowed lock (thanks Simbin! :/) of 23, the ideal rotation would be:
2 x 23 x 15.4 = 708.4 ideal rotation.
If you just want roughly the right feel for GT/Touring cars, then start off with 540/23 and if that's too snappy, go to 640-680 or so. If the rotation of the in-game wheel is set, to, say, 270 degrees, you can use the equations above to work out the ideal steering lock value for that rotation.
Hope that helped.