What should my voltage gauge be reading at idle?

if the gauge is right then the middle is supposed to represent 14 volts.the best way is to get a handheld battery checker and check it then.

yep. at idle, you would theoretically like to see 13 volts, so you are not discharging the battery. i know i sure dont. a real gauge can help. also different areas in the system will show different voltages. you may have 13.5 volts at the battery terminals, but only show 12.7 volts at some circuit in the car.

just check it with a DMM at the battery terminals.

good luck.