If you really want to check that out I would do some basic things.
Take a car with stock settings FFB for a ride and check first how the stock FFB reacts to flat spots, worn tires, tire temps, rubber on track, weight shift and so on.
I would do one after another.
Then adjust to your liking.
There is a nice circle at LRP where you can drive around as much as you want in the infield. I would take the Skippy for a ride there cuz there you can feel nice how the FFB changes if the rubber on track increases. Do 40 - 50 laps (it's just a small circle), slide around, check out how it reacts to tire temps, drift bla bla bla.
If the wheel rattles and jumps to much increase the FFB smoothness or however it is called. Go offtrack adjust again.
If you don't feel enough increase the FFB mult. But take care there is quite a lot of FFB clipping if you increase it too much. There must be somewhere a tool to see this, ISI forums I guess.
G27 here, smoothness on 2 for GT cars, Skippy, Clio and Megane. OW's not driven for a while but I believe I had it higher there, about 4.
Keep the FPS as high as possible!!!, very important to me and quite a difference in feel and driving.
Do long runs, adjust setup (can have a big impact to the FFB also) and have fun.