Just tie a piece of line to anything as long as that anything is part of the trapeze line. With this extra line you can pull the trapeze line to the bridle fastening on your hull. Then slide down your thumb to the fasteners while keeping the trapeze wires and line tight. Hold this position when you walk back to a given point on the rear of the hull. Move you thumb over the hull till the line is tight again. This is often near to rear beam. The distance measured between this point and teh rearbeam is a measurement of rake that is often used to determine the optimal setting of mast rake.

70 to 80 kg is the weight of a normal sized adult person. So just hang on the trapeze line from the stern and have your crew check the mast rotation.

Wouter