Not sure how exactly mast rake is measured on the prindle using the main halyard.

On the T we take a trapeze forward, tie a bit of extenston line to the trap handle and with the mast straight front-to-back, we pull the extension line & trap taught at the inner gunwale bridle tang. We mark/grip the line at the intersection point with the tang at gunwale (forward edge)...then walk back to the transom keeping the trap taught to check where the mark intersects the outside gunwale. Pre-spinnaker days (classic rig very similar to Prindle 18-2 & 19) the recommended location was 10 cm behind the rear beam. With spinny, we now run it typically right at the top/outer corner of the transom for most conditions...sometimes 2-10 cm down the transom below deck line.