LostinSpey· Please don't stand behind
What works well for me is the eye test (read about this on this site somewhere). Spool up some backing and then take the spool of running line and place it on (if you remove it from the packaging) near your reel to see how much space it will take up and also with the head. You are estimating the amount of space each line will use. Its a guess but has worked very well for me and shortens the time spooling up a reel without doing it a few times.