Basically all you gotta do is weigh the tank when it is completely empty then you know that when it is full, it will be 20lbs heavier. Then just go from there, I don't have a big tank but I play paintball and I know about filling tanks.
-Max Black 1996 XJ Build Thread: My Maine-iac XJ part deux
4.5" lift of assorted brands, 33x10.5 KM2s on D-Windows, JCR rear bumper, Nates Slimline w/Champion 8k winch, custom half doors, roof rack, and tire carrier, Maine Woods dents and pin striping, and various other cool things.
www.maine4x4.org Maine Jeep Club Member #1 Firefighter Jeep Club Member Black Jeep Society Member #163Survivalist Jeep Club Member #14