"Measure Twice, Whack Once!"
I vote with Morgan on this one...
Run the end of your hundred-foot tape up the main halyard shackle, but do tie a piece of light line to the shackle as well -- that way you can retrieve the halyard even if the tape should come loose. Never ask how I know this.
You could then hold the tape over the side by the dock, down to the water. Now you have most of your number. Then make a conservative guess as to how much further it is from the sheave box to the top of the antenna, using some binnoculars and your calibrated Mk-I Eyeball.
(Probably another 3 or 4 feet?).
FWIW, in a close situation with the underside of the bridge, I would be sure that my $40 Windex and my $400. windspeed/direction transducer would clear, even if the top of the shorty antenna bent a bit......
Now you will have the "air draft" for your boat. So, whether you have a short rig, tall rig, or mystery mast height modified by some unknown prior owners, You KNOW what you have.
Write this down on the cover of your Log book in big print, along with your call sign and any other basic stuff you might need in a hurry...
Hope this advice is worth... what you paid...
Loren in PDX
(No hurricanes, but we did have a typhoon once... Oct 12, 1962, and we aint forgettin' it real soon, either!)