If you have full rolls of the same material could you weigh them both and figure the remaining material from the difference?
On the other hand, you could give this a go:
https://handymath.com/cgi-bin/rollen.cgi?submit=Entry
Cool, I didn't know this existed - I created my own formula for doing this, mine is pretty darn close to this calculator, but not exactly. My method does work well - I do it all the time to see if I have enough on a scrap roll to do a small job.
This requires a digital caliper
1. With the digital caliper, set it to mm. Measure the Thickness of the material left on the roll - minus the core. (call this number A)
2. measure the thickness of the material - just one layer this time. (Call this number B)
Divide: A/B = C
Now take C and multiply by 11 (the usual diameter of the core). Divide that by 12 = equals how many feet are left on that roll. I always subtract 3 (ft) from that for a little cushion.
I really like that calculator you shared though... I'm going to book mark it and try it!