CO2 is sold by weight, not fill level; e.g., I have a 20 lb canister, which is 20 lbs of (mostly liquid) CO2. All of the canisters I've seen also have a tare weight (weight of the container) stamped into their sides. So you can measure the fill of your can by weighting it and subtracting the tare weight.
My gauge reads in psi, at about 800 psi new and stays steady as the level decreases. When it gets close to empty, it'll drop fairly quickly to maybe 200-300 psi. I keep a second 5 lb can as a back-up, to use until I can get the 20 refilled.
I also keep the gas connected constantly. And I fully agree with HoosierBrew's liberal use of keg lube - cheap insurance - and the 30 psi seat-the-lid process.