Summary

Filament ruler is designed for my spool(3D Prints! PLA 1.75mm 1KG; D_out=199mm,D_in=80mm,W=66.5mm). The packing efficiency is assume to be 75%.
Instead to measure D_core, just the distance between core and spool edge is used. The procedure is thus simplified onto a filament ruler and visualized results can be obtained.
Circle symbol is for filament length 100m, and square symbol is for filament length 25m. Type 2 is for another spool(FLMT PLA 1.75mm 1KG; D_out=200mm,D_in=80mm,W=60mm). For other kinds of spool, you can use customizable filament ruler (https://www.thingiverse.com/thing:3886663).