I really admire who is able to notice wich setting is the one to change at given set up.
Sorry there is no fast rule, I think the right comparison is that car tuning is a black art.
I don't know if it was Prost or Senna that wrote that there is no cartesian way to setup a racing car, it is trial and error, based on your tuning experience.
First you must understand all concepts explained in the guide, and the inter relation between them, that is the easy part.
You know now the basic rules of music theory, can you play piano now?
Now you must choose an esay piece of music, and work on it.
Translation, take a gt car with little aero downforce, Nissan 370Z for instance, high downforce cars can mask errors and introduce an added complexity, on a track you know perfectly.
Choose half tank (when all tuning will be finished, try with full tank and adjust if necessary).
Run with default setup until you are unable to improve your lap time, and that you are able lap after lap, to run in the same segond.
Use the tire widget and observe the evolution of temps as you lap, adapt you driving style to avoid understeer in corner entry (kills front tires).
Are you heating your front wheels much more than rear tires ?
Then observe the evolution of your tires, are you braking with front wheels only ?
Use a wheel or keyboard key to adjust your brake balance.
Adjust gearbox to reach max rpm in the longer straight, and distribute gears accordingly.
Even if going slow in corner entry do you heat more you front tires than the rear tires?
Try to cure understeer applying the recipes you have found in the tuning guide.
Most cars understeer on default setup to make them easier to drive for newbies.
Are you using now the four wheels of your car, and not mainly two?
Are you faster on a long stint now?
Maybe you discover that the car which was so easy to drive with the standard setup, is now much more difficult to drive...
Then stop your tuning, and next week try the car again, are you able to reproduce the lap times?
Is the balance still to your liking?
After tuning a lot of very different cars, on very different tracks, slowly you'll begin to improve.
Remember there is no perfect setup, every setup is a compromise, and different drivers drive with different setups suited to their driving style and capabilities.
Avoid applying recommendations (this car requires xxx pressure) read on the forums without checking them by yourself. I have tried many setups downloaded from internet (may be 50, and I only drove one which felt good to me -from David (davidporeilly) by the fact-). Many drivers are able to set up incredible lap time with really awful setups.
After many years of car tuning, I often find myself in a situation where my car is worse and worse after each improvement I apply, so I close my session, and next week I'll start from standard setup again.
You won't be able to avoid the fact that a pianist with 20 years of continuous training is better than an other with 3 months experience.
Cheers.
.