Here's another shot of one on a Vanessa:



Note at the wingtips the piece of string held on with tape. That "drew" a CG line all the way across the wing, as a point for me to aim for.

FWIW, the Vanessa cost me roughly $10 to build...and the rope was a majority of that. *heh* They do, indeed, seem a little "confusing" at first, but once you get the idea, they're really pretty simple.