@mesfigas Ignore throttle input; your FFB will hit its biggest values when you are cornering, especially in fast corners with winged cars (they generate higher forces with the extra downforce). But what you'll also get is little spikes from bumps, kerb strikes, contact with walls. So the best way generally is to do a lap or two, take a look at the FFB output graph and see which part of the track it's running highest at (not a spike, but staying at a high level for a second or two), and when you do a fast clean lap through that section you can check the FFB to make sure it's not sitting at maximum and not moving, but is very close to full.
If you try to 'keep' all the spikes you'll end up with a very low multiplier and weak FFB, while if you overdo it you will be driving some sections with your wheel just pushing at full strength - which doesn't actually give you any feedback, which is obviously the point
Once you get to know the car you're in you'll basically know which corner to check the FFB on. Just be aware that if you set it just on the limit with low fuel, it might clip a lot with higher fuel, as the extra weight can give you stronger FFB. Caster changes are another one to watch, big effect on FFB strength from that.
As an aside, I was going to try and make a simple plugin that would monitor the FFB output only and give a message with the suggested FFB multiplier if the current seemed wrong - but plugins can't see what the current multiplier is, so it wouldn't work well. It would definitely be better if the game had a working FFB meter built in, and/or some self diagnosis that helped correct the value. Considering the game obviously runs the physics, I'd prefer a simple FFB mode for consumer wheels which takes a calculated guess at maximum steering column torque and automatically adjusted the FFB mult to suit.