reading a trim gauge

[dakilmartin1]

i just installed a new trim gauge on my donzi f-19. it appears to be working when i trim up and down on the trailer. my question is what am i looking at? the plane of the boat? the angle of the motor in comparison to the hull?
i am a newbie boat owner but i do understand things mechanical. i built and raced stockcars for a long time. if the needle is up what does it mean and visa versa. we didnt have trim gauges on racecars we took measurements from a flat surface:checkered::checkered:

[BlownCrewCab]

It's more of a reference point. Not any particular angle. When you have your boat running at a trim setting that you like-Look at the gauge and next time your out you'll be able to return the trim right back to that sweet spot by referencing the gauge. also It show whether the drive is trimmed up (from trailering Or Shallow water) so you'll know to trim it down and Vice Versa.