Needed to print something that slicer estimated would take 80 gram of filament. The PETG roll was almost finished, the weight of the entire spool including the paper roll was 200 gram. I measured an empty spool to get the weight of the paper roll, which was 120 gram or so. Subtracting that, there was exactly 80 gram of filament left in the spool. Started the print anyway and came back to this in the morning.
Lol, that was close.