TRHit, new Stalker Sports are sealed and have no user accessible calibration. They come with a tuning fork and also a "test" button (which should show "80" when pushed). Stalker says to return the gun to the factory for adjustment if the user thinks the gun has a problem.
Regular calibration isn't normally required. Highway Patrol guns are regularly calibrated because their readings are frequently challenged in court. They receive heavy use and are often used in adverse weather.
As we all know, even new quality guns show nutty readings. With identical guns, one may read 3 mph high on one pitch and 3 mph low on the next with the gun in the same location. (complex subject has been discussed many times on HSBBW but always interests me)
I doubt the problem is often with calibration. That's also why it's hard to know how much difference pitch location makes on the radar readout.
Pitchers should be gunned over many pitches to get a proper indication of their speed. I THINK low pitches may be faster than high ones, but it would take some large scale research (say, by PG) to know for sure.