their is no ideal pressure.
it depends on the shock, the frame, the rider weight, and the conditions you want the shock set up for.
reitterating the preveous statment, your looking for 20% sag as a rough estimate, guess your pressure, sit on the bike, bounce, let the suspension push you back up, and then measure how far the shock is compressed via the o-ring, adjust air pressure untill its just right.
if your still none the wiser, find somone who is, best place to look being your lbs.