first I have to say this is the way I do it. I don't know if it works well for you but I think so.
1. Get an good setup and drive as fast as you can.
2. Take a drivetrain from another track where you think it's nearly as fast as the circuit that you are running.
3. Look when the engine come into the RPM limiter.
4. Look at your highest speed. When it's just a bit slower than your drivetrain can run it's okay but if you reach the limiter on the half of the straight you have to try a longer drivetrain. If you can't reach the RPM Limiter you have to try a shorter drivetrain. Also if the speed you reach is much lower than you can run with the drivetrain you have to do it shorter.
Finally I say:
If you come out of corners well and you don't reach the RPM Limiter for a too long time the drivetrain is okay. A plugin which shows you a RPM curve of the hole track can be very usefull.
This is the way I do. Correct me if I'm wrong.
Thank you for doing this, its basically right. But you can do it easier. Just try to reach the rpm limiter at the end of the longest straight of the track. Stay in rpm limiter for max. 2 secs or something before braking. Sometimes its faster to stay in rpm limiter longer (f. ex. Kerpen) or shorter, depends on the track. Not the best way, just my opinion.
That's why I try everything. I trained in Kerpen today snd also on tracks like Wackersdorf it's better to drive shorter. But you don't know it. That's why I think it's better to do the complicated way. But I don't know it sure. :)