Maybe there's some technical reason why a "real" fuel/pressure gauge can't be used? The thing is not operating at STP, and
throw in the orientation, micro gravity, and speeding around at roughly 18K mph, in the sunlight for say an hour, behind
the Earth for maybe half an hour, it does present a challenge.
Cheese Whiz.. ( not a commentary, but an example.. ;o)..)
Products in pressurized cans are contained in a pouch..the "propellant" is a pressurized gas in said container, surrounding the pouch..
The gas pressure in the container will decrease as the product, ( cheese ) is dispensed from the container..
The pressure of the gas in the container can be measured, indicating exactly how much product (fuel) is still remaining.
The Apollo program landed american astronauts on the moon and returned them back home again 36 years ago. Even Apollo 13 was electronically rescued, has "faster, better, cheaper" gone too far as NASA's philosophy since then? Actually though, AG was discovered/developed years ago, problem is : with everyone in the world having their own magic flying carpet, the entire third world would be landing on your lawn as a plague of locusts(like the mexicans are already).