Thanks for the suggestion. I will look into that. However, someone who responded to my post about this on Reddit brought up the point that a line unwinding from a spindle will not unwind the same length each turn, so measuring the turns of the spindle wouldn't be the best way to do this. They suggested using an optical sensor of some type to measure marks on the line that I make a specific intervals (1m or whatever). I've never used an optical sensor this way before though, so I'm doing some research on that. Have you ever used an optical sensor this way?