Fix it to the windows of your spool, it will help you to know the percentage of left filament.
If the gague dosen't fit your brand, you can:
Scale the model in slicing software eg.Bambu Studio, for example, my filament spool thickness is Φ90mm (170-80=90), if yours is Φ105mm (180-75=105), your scale should be 16.67% (105/90-1=16.67%), it's not the perfect solution, because the difference of ID & OD, but it's OK for you to know how much percentage is left.
just leave me a message of your brand information, I'll add a version for your spool. e.g. Bambu PLA OD170xID80mm
Features:
0.4mm total thickness
10% for each grid
use double sided tape to fix
Print settings:
0.2mm height PETG/PLA
IMPORTANT: Wait for hot bed totally cooling down! be patient!