Today we measured the mast bend for a new mainsail on a Sweden 370 and took a few shots. Here's how you do it.
First, hoist the main halyard to the top, but attach to it a measuring tape and a low-stretch rope, in this case, a 1/8" Kevlar. Tie-off the Kevlar at the bottom at the right length so you can crank the halyard and load-up the line, to make the line bone-tight with the halyard shackle at the top of the mast. Then, measure to the top of the boom. Record this number, 3/4 of this number, 1/2 of this, and 1/4 of this for later use. Here's a shot of the Kevlar cord very tight with the measuring tape a little loose in the breeze.
The Kevlar cord is the straight-line, the visual reference used to measure the mast bend. Make sure it's up against the aft face of the mast at gooseneck level, even if you have to tie it forward like this.
Then (on another halyard, of course) send another guy with a ruler aloft in a bosun's chair. Hoist him (or her, of course) up to the numbers you recorded before (not to the top). Then he's in a position to measure the mast bend at the 3/4, 1/2, and 1/4 points.
Do this twice: once with the mast with a loosely snug (but not too wiggly) backstay. Then do it all again with the backstay in the tightest position you'd have it while racing. The latter setting should yield larger measurements to reflect more bend. Record all these numbers, and it allows us to get the luff curve of the mainsail just right, creating the most efficient shape, and allowing you to use the backstay to depower as needed.