It’s covered somewhere, but I can’t remember where. In essence, your thumb really shouldn’t be doing any work behind the neck, i.e. it shouldn’t be exerting any force and you should be able to lift it off, without affecting your fretting. In terms of your fingers, they should be applying just enough pressure, just behind the fret, so create a clean, buzz-free note. Any more pressure than that is wasted effort. The simple exercise to do here is play a note, gradually increasing and decreasing pressure, you will hear, and feel, the sweet spot where there is just enough pressure.
Of course the difficulty is maintaining this as you play a baseline!!! I was playing one last night and ended up really tense, holding the neck like a baseball bat. Needless to say, my playing was cr@p. Relax, ease off and bingo, I could hit the notes cleanly and play absolutely in time.