Final drive is as it says the final ratio of the gearbox, that affects all the gears (multiply ratios).
So:
If you want to keep the ratio / cuve between the different gears but want a longer / shorter span, keep all gears the same and change only the final gear ratio. Example: when you start working on a restricted car like GT2 it is an easy way to take a GT setup for the gear ratios and use this slider. For fine tuning you will need to read below.
If an (some) individual gears do not please you (for ex if the layout is such that you have to change up just before a corner) then change only the gear(s) concerned using only the gear slider.
Optimum is not possible because it depends on the track, wind, other setup options and driving style. You can get a good guess using Bob's VHPA - check the unofficial addons theads.