When it comes to gauges and what they're supposed to represent there aren't too many I trust.
The best thing for you to do is put the boat on a trailer (or blocks ) and energize the trim gauge and see what it's showing when the drive is level with the keel . Maybe even mark the gauge with a dot with it showing that position . That way you'll know where true level is.
But to be perfectly blunt any serious boat driver rarely looks at the trim gauge anyway as they drive by feel and I can assure you that any of them will know the difference between negative trim , neutral trim and positive trim .
If you've been operating all this time with the drive trimmed all the way in you've got a bit of catching up to do . Didn't anyone ask you why your boat was always running bow down ?
Just because something's old doesn't mean you throw it away !