Depending on car and personal preference, a value between 25% and 60% ingame FF is good. Any less is too weak, any more and the wheel just becomes very heavy and kinda "on/off" because the forces max out early. Basically the lower you go (up to a certain point) the greater the range of forces you can feel, though small forces like rumblestrips or grass can feel very weak. The higher you go, the more pronounced the small forces around the centre of steering will be, but as soon as you overturn the wheel a bit and start fighting the tyres all you'll feel is a constant pull into one direction, since the FF motors will work at maximum power and all subtle changes or vibrations are lost due to clipping. With high FF strength you'll also have to make sure to never let go off your wheel, or it will very likely start oscillating around the centre, caused by the lag/slow reaction time of the FF motors.
To verify your FF is working correctly, do two things
1) Get the car into a slide (oversteer) and let go off the wheel - it should automatically try to countersteer and correct the slide.
2) Flip the car so you're lying on the roof - there should be no active resistance to turning whatsoever.