newbie trying to understand MT50

my xantrex is set up through my shunt.
I am trying to understand how the Ah reading works, that shows up on my xantrex also but I dont have a clue what it means or how to decipher the meaning.
I have a 255 Ah battery, what should the AH reading be at full charge? somewhere close to 255

I'd expect a solar panel in a garage on a sunny day with door open to put out a few volts. I can get 10.5v from mine parked under a streetlight.

The panel does not have to be in full direct sun to produce voltage, and might even produce near its max voltage in cloudy skies, it will howevernot come close to producing its short circuit current unless cold and in direct sun.

The MT50 to my knowledge does a lot of inferring as to battery state of charge, by guessing after looking at voltage. it is not a real battery monitor so take its readings as to battery state of charge,with a huge grain of salt and more than a little skepitism.

I have a Shunted battery monitor and completely disregard the battery % remaining screen, and only trust the Ah from full screen if I have recently reset it when I have verified full charge by other methods.

Exactly.

Hi Tom, ok, you mean when I have it outside on a good sunny day use my voltmeter and try to touch the connection where the positive and negative panel wires are entering the charge controller?

I wouldn't make any changes. There may be some voltage bleeding back through the controller. When you are back out in the sun, connect the panels and measure the voltage at the panel input contacts and compare it to the remote reading.