Gauge pressure

I think the easiest thing would be to get something like the EdGun EaZyfill, or one of the knockoffs, and use that. With it, when you open your tanks valve the line will pressurize and the tank's gauge will read what is in the tank. Then use the fill device to control the airflow into the gun - but then you'd have to read the fill pressure off the gun's guage as the tank gauge won't reliably show the gun's pressure (and will pop back up to tank pressure when you release the EaZyfill valve.

Of course you could choose to only use the EaZyfill when you need to read the tank pressure . . . but the cheapest way to go is to get a dead head fitting and use that to read tank pressure.
 
I think the easiest thing would be to get something like the EdGun EaZyfill, or one of the knockoffs, and use that. With it, when you open your tanks valve the line will pressurize and the tank's gauge will read what is in the tank. Then use the fill device to control the airflow into the gun - but then you'd have to read the fill pressure off the gun's guage as the tank gauge won't reliably show the gun's pressure (and will pop back up to tank pressure when you release the EaZyfill valve.

Of course you could choose to only use the EaZyfill when you need to read the tank pressure . . . but the cheapest way to go is to get a dead head fitting and use that to read tank pressure.