My two cents... I don't know about any type of fancy algebraic equation to figure it out but I have found it handy to create a easy measuring guage. Take a full 50 yard roll and measure the diameter, unroll it at logical increments (10', 20', 20' etc) and measure the diameter at each interval. Then you can use 1/8" clear acrylic, D/S tape it to a trimmed core end and use vinyl to create multiple increasing circle sizes radiating out from the core that correctly gauge the remaining material. Though it takes a bit of time up front it is very useful and quick to gauge a roll after you have your template done.