What I describe here is for a 6-Speed Full Custom Transmission. Some are 5 or 7 speed, so change as appropriate.

1) Set the Final to about 3/4 of maximum and make note of the number. I generally use the closest whole number for convenience. This should give a nice range for adjusting top speed on fastest tracks, and best acceleration for slow curvy tracks, using top RPM of 6th gear for fastest speeds atainable.
2) Set Auto to 1. If already on 1 or 2, move it away from 1, then to 1.
3) Set first gear and top gear to maximum
4) Set second gear to minimum
5) Set 3rd to 5th as needed for the mild curve normally seen. Optionally, set the bottom of the lines to a flat shape. Linear (flat) is generally best if using automatic shift, exponential (curved) is generally best for manual shift. This however varies from car to car. The key factors that determine this I may explain later, but mostly has to do with the power curve shape, peak RPM, and RPM limit. If using Auto-shift instead of Manual, where the Redline is plays a factor too.
6) When in the car settings, press the START button to expand the graph. Note where the Peak HP RPM and RPM Limit is. The Limit is the number at the bottom right of the graph. The Peak HP is not always correctly represented in the car stats. If you can see the sections of flat lines making the curve on the graph, they are in 500 RPM increments, and you can closely approximate the HP Peak RPM. Also note if the shape is symmetrical at the peak or not. I will try to go into more detail of the importance here later, but need to attach pictures to explain properly.
6) Verify the starting point is right in step 1. To do this, go to the Driving Park. Use the Test Course and find your best Final Drive value for your best Top Speed. Try different setting of the Final Gear. Normally, your top speed will be best when using the Peak HP RPM on the long curves. Adjust with your discretion for either top speed or best lap time. They are not always at the same point. If the Final cannot be set low enough to achieve this, the starting ¾ value will need to be increased to achieve this range. However, check the next step first to verify it can be in range too.
7) Now go to the Beginners Course. Start with a high value for the Final ratio. Many cars are powerful enough that minor changes make no difference on short tracks. For the cars that do, your optimum point will generally be 6th gear just past the redline before you have to brake. Just verify you can set the final value high enough to redline or limit in 6th gear. If not, you may want to start with lower ¾ value in step 1. High power cars, you may want to not go past 4th or 5th gear as the more you shift, you lose time the tires apply power to the tarmac. This check normally only applies to cars under, perhaps, 600HP, but again, varies by car too.
8) If the full range is not achieved in steps 6 or 7, start at step 1 again with a lower or higher value. I have not yet seen a car that cannot be set for both extremes in the Final gear, but I assume it is possible you have to compromise.
9) Never make auto-set changes from this point, only adjust Final Drive for optimum max speed for the track. Set final drive to lower numbers for high speed tracks and higher numbers for max obtainable speed for short tracks with no room for attaining high speed. If full range not attainable for slowest to fastest track, then the 3/4 point in step 1 may need a starting point higher or lower. Use the test tracks for Top Speed and 400 meter to test if you have full range adjustment with the final.
10) Normally, you want a shift point that is a balance on each side of the peak RPM for best accelerations. Example.. If you have 7500 peak HP and a symmetrical power curve above 6000 RPM, redline at 8000, limit at 8500:

Shifting at Redline like Auto does, may make the next gear start at 6000 RPM. This is not balance. Shifting at the 8500 limit will make the next gear start about 6500 instead. This will be more effective for acceleration times.

Best points are hard to explain to cover all cars. This is something to experiment with, but keeping the graph points in mind, you normally want the power at RPM shift points to be equally distanced from the peak HP RPM. Sometime the Limit RPM keeps you from doing this.

I have also noted that if you want a one-size-fits most setting, use the final setting that gives you the best 0-1000 meter times.

If you want to set the gears for an equal RPM shift, then do the following after Auto-1:

Set 2nd gear all the way left
Set 5th gear all the way right

Subtract the numbers of each other It will be around 1.7
using a scientific calculator, take the 3rd root of theiir difference. It will be around 1.2

4th gear will be 5th gear times root number
3rd gear will be 4th gear times root number
6th gear will be 5th gear divided by root number
7th gear will be 6th gear divided by root number

Final can usually be started full right and get full range.
1st can be left default, and with high HP cars, doesn't play much effect anyway.

This will improve the accelerations for many cars! Just remember, touching the auto slider will mess these setting up. Use the Final to select your range for each track.