Hi Tim and the rest of you.

What I do to measure mast bend is:
First you will need:
1 tape measure long enough to go the length of the luff (or you can mark the mast with texta or pencil every 500mm starting from the top black band.
1 tape measure or ruler up to 300 mm in length.
1 string line long enough to go from the top black band to the bottom black band.
1 note pad and pen or pencil.
1 helper makes taking down the numbers easy.

Rig the boat as usual with normal rig tension etc..
Tip the boat on the side and rest the mast on something around the hound height.

Adjust mast rotation so that mast is as fore and aft as possible.

Scary bit number 1: Pull luff tension on as hard as you can.
Scary bit number 2: Pull mainsheet on as hard as you can.
Scary bit number 3: Repeat the previous 2 steps.

Your sail should look as flat as a board.

Next either mark the mast every 500mm starting from the top black band or run the long tape between the black bands.

Then run the string line so that it is tight from the back of the track black band to black band.

Then using the ruler or second tape measure measure the distance from the back of the track to the string line at all of the 500mm points. 0 should be the first number.
You should be getting around 250mm as a maximum give or take 10mm.
After you have all of these measurements you are ready to talk to the sail maker.

I hope this is a good enough explanation. If not keep firing the questions.

Just so that it clear, I do get helped out by UK Halsey sailmakers.
I do not get free sails, just a discount.
One of the reasons I use UK is that he is very keen to get feedback on his sails so they can be improved. He also has a great knowledge of the materials available and uses computer cutting equipment for consistent results.
I think that the other sailmakers mentioned are pretty good also.

I will dig out some pictures of mast bend if I can find them.

Chris Dean