I have a heavy duty office supply clamp fixed to the back of my workdesk. When I'm pulling a reel, I clamp the end of the string with the clamp, then pull the reel to a piece of tape I have previously marked with some common lengths. Cut. Easy as pie.
For single sets, I have a small eyelet screwed into a door frame. A slip the end of the string into the eyelet, then unravel the set. When both ends are equal, you know they are both 20' (or should be). Proceed as needed depending upon what you need for the job.