I had always assumed people did the following:
From http://www.rrtc.net/book/cal-layout.html
You needn't actually use a spring balance this way while taping your calibration course; it is enough to do a few trials beforehand to get a feel for the correct tension. With a little experience, it will be easy to judge the proper force, and you'll be able to dispense with the spring balance entirely. Note: It is acceptable to answer question 17 on the "Application for Certification of Calibration Course" by saying you estimated the tension "by feel."