The easiest way to measure muffler pressure (and still use it to pressurise the tank) is to put a T piece in the pressure line, run a length of clear tubing from the T and drop the end into a tall glass of water. Start the engine and run it at whatever revs and you'll see the level of water in the clear tube drop. If the tube is taped to a ruler you could measure the exact amount the level drops. Now 30 feet of water is near enough 15psi so 1 foot is 0.5psi and 1" is about 0.042psi. You'll most likely see it drop about 6" which would mean the muffler pressure was 0.25psi. Another way is to just raise the tube out of the water until you start to see bubbles coming out the end.