I find the best way to get accurate tyre pressure is to pump them up with one of those big bicycle pump to when the gauge is at about 34-35, then I attach a proper pressure gauge and let out until accurate (I run 34 at back and 32 in front).
I find also that the gauge on the pumps are completely useless.
I check mine weekly and usually need to adjust slightly every other week.

Also, mind you that the pressure will change according to the temperature.