New to Forza, help on tunning and drifting

I have never played forza before so when I did I started with all the assist on. By day 3 I removed most of the assist and converted to playing with manual and clutch. One thing I don’t understand is the tunning for the vehicle, how does anyone know the exact numbers? Is it just trial and error and spend hours on it or is there like some formula everyone follows?

My next question is whats the best way to drift in the game?
I am also new to xbox, xbox one being my first.