Howdy,
Those trim gages are simply a standard
[URL="http://www.engineersedge.com/instrumentation/electrical_meters_measurement/darsonval_movement.htm"]D'Arsonval[/URL]
meter at the panel and a variable resistor in the drive.
Because the variable resistor can get wet, corroded, or fouled with "stuff" the repeatability of position and display is not very reliable.
They're pretty good for telling you you're all the way down (and you verify because you usually hear a clunk)
And they're pretty good at telling you you're all the way UP (and you verify because the pump slows down.)
When you're running on plane, your range of operation is based on performance but it's a VERY small range where it works good. The gage is pretty much useless for repeatability.
(I never look at mine except when I want to know for trailering and I want it all the way down for holeshot)
You can usually do ball park adjustment by turning the sender CCW or CW on the drive if it's mounted with slotted holes. I can't remember if mine did on my old Cobra. It didn't work anyway.
