Specific output ratio.
I.e if I want to use a ratio of 0.5 I can set the overall strength in True drive to 25Nm and the Max force in game to 50Nm or 20/40 or 15/30 all three will give me a specific output ratio of 0.5.
Depending on the car and track I can use any of them without clipping.
Witch to use and why?
Clipping is caused by ingame force, so depends on wich car are you running.
F.E:
F3 will clip below 50-60Nm
MX5 below 15-20Nm
You can use the “clipping notifications” checkbox in one of the True Drive tabs, dont remember what, SC2 will beep every time the game sends 100% forces to the base
Edit:
And to really answer your question, if the game sends 100% forces (in normal driving) when TD is at 25Nm, can your arms tolerate the torque?
Im a weak driver, so no more than 12-13Nm on True Drive and then I try to maximize ingame forces without Clipping beeps
The number to worry about with regard to clipping would be the Max Force Number…
Generally speaking all three options are the same with regard to strength at the wheel.
The lower max force options lower the headroom you have available for the car telemetry to give you stronger information so that information will be clipped. There can be some useful information here in regard to catching a car that is out of shape but sometimes it is really just brute force. The Lower MaxForce/Lower TD Strength also has the effect of limiting the maximum force that you will from the wheel, which can be good in some cases.
The higher numbers guarantees in most cases that you will always get full telemetry to the wheel from the game… This does have the effect of also allowing maximum force hits to come through the wheel which can be abrupt…
Basically though if the car and track combination does not clip you can use any without issue.
Which option to use in my mind has to do more about how you use the sim… If you are jumping around from car to car from high to low strength cars you really don’t want to be having to change settings all the time so by running a higher Max Force with more TD Strength allows you to just switch cars without ever worrying about clipping.
If you use ONE specific car and do not like being jolted at all then running a Max Force right up against the Max Telemetry you get from the car and then setting Specific output as you want it. Then that is also good as well.
It is mostly about understanding where you want clipping to be, How Much strength you want at the wheel, and how you use the game.