Paul,
When working with the "x" measurement on the luff,
how was this determined?
is it the same for all sailboard masts?
do glass fibre masts and carbon fibre masts flex the same?
I just picked up 2 masts on ebay "cheap as"
IMO :- Glass Fiber masts and Carbon masts will have the same flexure provided all the physical properties are the same. However, the physical properties are totally different. Carbon has a very much higher modulus of elasticity and will therefore never have the same flexure. The carbon mast will be very much stiffer given the same stress loading 
Kody
I laid a sail flat and measured it actually. The sail was the best one I have yet bought, a Ken O'Brian sail from SA. The luff was recut flatter by Deiter Drabic ,a sail maker in Esperence, who specialized in sailboards..
I recut and sewed a sail and tried it . a few times I had to pull the top400mm apart and add a bit more curve if there is a wrinkle or if the the head wont stand up.
the carbon masts are stiffer, so they suit a chassis with a bit more flex in the main beam, and the opposite with the glass mast. if the rig is to be unstayed , I fit 600mm of 48mm od aluminium up the base to strengthen it .this comes from asupply of old broken masts and booms that have accumulated over the years.
I glue it in with silicon to give a flexible attachment, and cut fingers in the top of the pipe to prevent breaking on the termination of the pipe .
first sail Im recutting for the minis is a Gaastra 4.0 wave sail. because the foot is quite high on this type of sail so I will be adding to the foot instead of cutting stuff off.. Here is the sail with the pocket,wear tapes,excess batten pockets,and the excess re-enforcing from one of the eyes.
next I mark the new luff curve with the template. the new foot angle/position has been marked with rulers.
this sail has a lot of re-enforcing on the foot which has been re-used on the foot of the sail . the new foot has been shaped to give a bit of a "shelf" effect when tensioned on the boom. Here is the finished foot, and includes scraps from other sails. I have a supply of really old (early 1980's) sails which I save for the sail cloth and pocket material.
The unpicked pocket pieces are joined together for the new pocket and resewn. If it is to short I set it 300mm short of the head and 300mm short of the foot.
The new eyelets are put in by myself. I invested in a really good punch set from my local helpful canvas lady who has been a great help from day 1 when I asked her to sew a sail back in 2005.. she also supplies the eyelets parts and when the sail is too thick she will do the smallest jobs at a moments notice.
THANK YOU JACKIE from KALGOORLIE KANVAS.
here is the almost finished sail laid on a 4m blokart sail. notice the slightly different angle on the foot. the luff curve is very similar, with a bit more curve on the head to tension it better
the next sail is a hood 4.5.again a surf sail witha high foo. theis sail is all soft, ie no monofilm, I will build this one with a set of reefing points. it should measure4.2/3.3m when finished.
as before, remove all the fruit and measure and mark.
the xtra sail added at the foot is from a late 80's sail that was mostly U/S from the mylar separating. the lowest part of the foot has been removed.
more news to follow
once this sail was recut and resewn I fitted it to the softer of the 2 3 piece masts that I have made. the photo of the sail shows that the luff is still to round for the mast and rigging system. the stuff up is probably due to the original sail having a well rounded roach .With recutting that has helped to alter the luff round .
the sail now has to have the mast pocket removed and the luff recut ,again with the template . you can see just how much the next recut will take off the luff.
the sail has been resewn and tommorow I will fit it back on the yacht. cross your fingers
now thats out of the way, the next step is to build a sail from te ground up. keep watching as I started that little project yesterday
I took this photo of the sail underway to give an idea of the sailshape you are searching for when its all recut. the wrinkles at the lower batten pocket will be pulled out when I tighten the ropes at the foot 
the top batten is slightly to stiff and could be softer forward.. here is the sail underway. nice mast bend and twisting away from the centreline just nicely 
sail is not yet sheeted in for sailing at speed
