How do you correctly measure mast rake? I've read various references, most notably the Quantum tuning guide. Hoist measuring tape with main halyard, pull forward on forestay, should be 21' 10-1/2" to 21' 11" to top of transom. What I'm unclear on is, where exactly does the end of the tape go? The bearing surface of the halyard shackle pin? The top of the halyard shackle? The top of the knot that attaches the shackle to the halyard line, so that the tape is all the way up against the sheave? On my boat, there is almost 3" between the top of the knot and the shackle pin. So it would make a big difference. Thanks!