Rule of thumb?

Well, who's thumb are you using? My thumb is likely different than yours.

The feel is a personal thing.
As a general rule you want about 1000 psi of pressure at the caliper.
Now to put a wrench in things...

When you have a longer lever arm (read larger diameter rotor) you won't need quite as much pressure. It's like having a longer wrench. You don't need to pull as hard to get the same torque.

That means you can use a bit less pressure with larger diameter rotors. What doesn't change is pad movement.
To get an idea how far your pads move, run the numbers on your stock stuff. Try to work out similar pad movement at a reasonable pedal travel and pressure.


Clear as mud?