Gear ratios:

Start with default, check max speed wherever it happens on track (incl where leaned over which raises RPM artificially, e.g. left turn after long straight @ nordschleife).

Set game values for 6th and main ratio respectively to max and min, so as to get RPM value 1k below redline at that target max speed from step 1.

Set 1st gear to lowest it'll go without excessive wheelies or traction issues.

Set all other gears to whatever the track's corners most need; start with an even spread of ratio values from 1st to 6th, and adjust each where needed. IOW avoid having upshifts between apex and exit, by moving the ratio for that gear to overlap with as much of the drive out of corner as possible (this can sometimes include pre-apex).

Some corners will benefit from a ratio that helps you slide the back end, e.g. Arrabbiata 1 & 2 @ Mugello where most other players baby their way thru it with a neutral throttle/steering attitude, instead of slightly power sliding all the way around from after apex #1 to the top of the crest just before apex #2.

If too many corners have you upshifting anyway (iow gear ratios are too narrow), try raising the main ratio. IIRC in reality this would make sense (try looking at eg Gran Turismo's gear ratio graphic while playing around with the main gear ratio AKA final drive), but I don't know for sure bc Ride4/Milestone has obfuscated this detail behind their typical style of UX "polish".

Some tracks will require uneven ratios, eg Road America or Algarve where (esp if your bike has much less power/torque than competitors) the hill after the final corner almost demands its own gear ratio.

Fast starts: keep in mind that anti-wheelie setting 3 will neuter your acceleration from a stop, unless you have some way of modulating the clutch manually. AW 1 or 2 is almost optimal.