I turn the car around at the start of a session, go backwards down the straight, then turn around at the mini karussell and head back, watching for peax in the cars power thru the gears and the top speed attained along with the amount of revs left to go.
Then i go to the transmission settings, look at the number value for the 6th or 5th gear depending on car, and use this value as a final point for the gearbox.
If i want quicker changes and more grunt, then i shrink everything using the auto slider until the 6th gear still retains its value, but all other gears are closer to it, or i lengthen it if i need longer gears or fewer changes until 5th or even 4th is at the value matching the original value of the top gear in the first place.
Once i have decided which gives a better top speed for the car at the ring (which is very important-lots of high speed sections of course) i tune and test and tune and test until i have a good setup for the tricky sections and the best top speed i can attain.
If you want an example, pm me.
For the arta, i lengthened the gears as they gave a greater top speed this way, then upped the final drive ratio so that the car "got there" faster.
Its a balancing act Goix, and only time and trial and error will give the best results and teach you the basics through time.

👍