Ed,
The rule of thumb is 10 PSI minimum per 1000 RPM. IE: 60PSI at 6000 RPM.
As far as the probable cause, if your pressure comes up to 40 psi just above idle and then stays there, you may be able to increase it with a higher pressure relief spring in the
oil pump.
If it gradually increases with RPM all the way up, you likely have excessive bearing clearances, or an internal leak in the oiling system, or an insufficient
oil pump volume.
If it actually decreases with increasing RPM, the most likely culprit is excessive rod bearing clearance.
My 427 runs 35-40 psi at idle, and tops out around 100 psi by 6000 RPM, remaining fairly constant above that.
Hope this helps,