The FFB settings would be different for each car and each setup. Suspension geometry and weight affect the FFB a lot, weight can also come with 'downforce' if the car has that.
Here's the problem with force feedback. We do not see the actual percentage of force feedback sent to the wheel. Thats why we can't tell if you need 10% or 40% or 100% in the LFS menu in order to truly maximize the force levels.
Set it too high and you'll get clipping; for example in a downforce car you wouldn't feel the difference between a 80 and 120mph corner, which you certainly would in real life due to the downforce. Set it too low and in general the forces might be weaker than desired.
Since the G25 is such a weak wheel, even though its probably about the best we have, I suspect lots of folks run too high FFB settings in LFS, just as it gives them a 'meaty' feeling. When a G25 feels 'meaty' though, you're likely to run FFB levels so high that even minute steering would generate 100% forces on the wheel.
Same for iRacing and rFactor, lots and lots of people running what is basically an on/off switch for force feedback. In LFS i rarely used more than 25%. With the Fz50 and some caster, even that caused considerable clipping of the FFB signal! In iRacing often the ideal value could be below 10.
At least rFactor with realfeel allows you to set the FFB levels per car, so that does require a minute of tweaking but then when you switch cars you don't have to change the FFB.
I've been on LFS and iRacing forums asking for a ''True FFB %'' meter, like a FPS counter so you can see when that number exceeds 100%, your FFB wheel won't deliver any higher forces.