Do this to find your proper pressure for a given tire. Put white shoe polish on the out side of your wheels at the edges. Drive around at the hardest you would on the street real quick. Not full autox hard but the hardest you would on the street. Start at recommended pressure and work up. The lowest pressure were you no longer get the tire rolling over is the right setting. You can tell if it rolling over too much if the shoe polish is worn off past the little triangles on the shoulder of the tire. Look at this pic of a MX from the rack do you see the little triangles on the outside of the shoulder of the treads. (look right above the A in Ecsta)
BIG view of tire

I typically take my favorite on ramp to work really hard as the what I drive hardest on the street test. It will take you some time but once you have it your all set. You also do this at AutoX to find the right pressure for your tires.