Are you sure that you have all of the other steering related settings adjusted in the appropriate way? In particular, the 'wheel turn' and 'wheel turn compensation' sliders in the <options>....<controls> page ? Do make sure that the 'wheel turn' slider is set to the correct arc of rotation for your wheel (180 degrees?). Settin the 'wheel turn compensation' to 1.0 should give you good linear response in the centre section of the wheel, which should address your problem, I think.
I've attached a setup which I created that I find very effective - I can lap the LX4 with this quicker than I can many of the much more powerful cars (FZ50 et al) on the tight circuits. I also use a Logitech wheel.
Anyway, hope that helps.