my friend has a large co2 tank that we fill off of but whenever we let him fill them they don't get as full as if we were to go to the pro shop. and the pro shop uses the same tank and tank setup as we do. im looking into buying a scale to make sure we fill them all the way.. but i don't reall know how to tell when they are full.. do i just weigh the tank. then fill it with the correct amount of co2? like 20oz