I really would suggest spending the $30 or so to make a decent tillering set up. All you need is a 2x4, a hand saw, a couple pulleys, a yard stick, some string and a luggage scale.
Don't even need a pulley, just a wheel thing.. but how to secure the board so the string can be pulled?.. pulley may be better anyway.. less force would be applied perpendicular to the board. I imagine it'd have to be nailed to the floor and wall.
I have some ridiculous block and tackle thing i bought laying around :\ with too many built-in pulleys... it'd still work perfectly, if it had less pulleys. Can just have the scale directly attached to the bowstring, then the block and tackle attached to the scale.