Low- tech ish: Cut thread into the lengths you want it to be spooled to. Then time how long that takes to wind. Repeat several times. Time your winder to run that length of time. Run a few batches spooled from longer string for the time period. Check the length on the spools would for the time to make sure they are the correct length.

Maybe add 5% length to make sure your customer are always getting at least the predetermined length, to prevent less than a full measure ever being provided.