|This reminds me of the ancient joke about the two (insert nationality you want to insult here) guys who are told to measure the height of a flagpole. One says, "It's really light - let's just lower it and run a tape along it." The other answers, "No, dummy, he told us to measure the height, not the length. You'll have to climb it." And he does.
This problem is a lot easier. The Evolution page gives us the I and the J for a Mark I (33' and 11.75' - call it 12' for simplicity), so by simple geometry, the distance from masthead to the tack pin is 35.2'. Add another 33' to bring the halyard back down to the deck (68.2') plus the length of the boom, E or 10.5', to bring it back to the cockpit plus a bit (78.7'), then whatever fudge factor you think necessary. Done.
And there I couldn't see the point of high school math!
C&C 27 Mk IV