Basically, it just has to be tight enough to turn the wheel , and not let the headset pre-load, slip out of adjustment..

the friction is what the clamping is, add to the friction with carbon assembly paste? .. essentially sand & grease.. combined..

note: we (Mechanics) have long used "Race Tight" for stems ..
tight enough to work normally, but loose enough to move in a crash , to not break so easily.

a good plan for the brake-lever to handlebar bar fitting too..