If you want to know the amount of media left on a roll and not depend on software to keep track, then you have to measure it. It's quite simple. If my innumerate associate can do it, anyone else can do it as well.
All measurements are in inches, centimeters, furlongs, or whatever, as long as each of the measurements are in the same units. Measure the media on the roll from core to top of media, call that media_height. Then divide media_height by the thickness of the media and backing sheet, call that layers. Then multiply layers times the core diameter + half of the media_height and then multiply the result by Pi [3.14159265].
In one swell foop: media_height / media_thickness * (core_diameter + media_height/2) * Pi.
If your measurements are reasonably accurate, no need to go completely anal here, the result will be a usable estimate of the media left on the roll in the units you used for the measurements.