Nick, with a properly operating charging system and a good battery the meter should show a fairly high charge after starting, then taper off to just lightly above the center (zero) position. If you turn on the lights or engage the PTO the meter should indicate a moderate charge, but a lot depends on the capacity and condition of the battery. Also, small tractor ammeters are not precision devices so readings sometime vary from one meter to another.
In my opinion the only time to be concerned is if the meter consistently shows a discharge with the engine running at 1/2 throttle or higher.
Stan in CT