Welcome to SBO, Thedroubies.
You have come to the right place for all things sailing. You obviously have a great sense of good sailing to make this decision for answers to your questions.
Others who know a lot more than I will be along to help. In the mean time, I would suggest two immediate solutions. One, as a former Maths teacher and design engineer, trigonometry. The other, run a 50' tape measure up your halyard and add the distance from water plus a foot for the mast head and weather vane to the measurement. That should get you pretty darn close.
It is great to welcome you aboard and good luck and good weather.
-Will (Dragonfly)