Sparker, the way I do it is just to run a pull-out rule around the outside of the pulleys, pull it firm, and read it where it reaches back to the tape reel. I am uncomfortable with your reference to rope. The mathematics should work though, for a simple two-pulley system: twice the center-distance plus half the outside circumference of each pulley. If you had another pulley it would become quite unwieldy, of course.
