It depends on the engine's powerband how much preload you will need.
It is also up to the driver, most experienced drivers will use more preload because it makes the car harder to drive, more aggressive.
I don't understand how you are able to burn out the thrustbearing, if you use quality grease it should outlast an engine. Probably too much endplay.
If you burn out a clutch shoe you are running too much spring tension, or the clutch isn't shimmed out properly so the shoe doesn't have full contact with the bell.