mainsail
Because your location is so remote, you really need to check the dimensions. Measure from the back of the mast (near the tack fitting) to a spot about 3" fwd of the outhaul block at the aft end of the boom. This will be your E dimension. Then measure the distance from the tack pin to the back side of the mast-this is your tack "set back"-about 1 inch. E minus tack set back =foot length. It is important to do it this way so your new main will not have a wrinkle at the tack.
For the luff, measure from 1" below the bottom of the main halyard block down to the top of the boom, near the tack fitting. This is the P to use in shopping for a sail. Then measure from the tack pin down to the top of the boom (about 3/4"). This is the tack cut up. Same thing as with the foot.
Now you have a P, E and the tack setback and cut up. Shoot for a 90 degree tack angle for good sail area and no boom droop. If you have a bimini to may want to measure for a max leech length my hoisting a tape to the top of the main halyard and down to the maximum level you want for the boom to clear the bimini/dodger, and subtract about 8" (for the boom height and stretch).This may end up less than 90 degrees, but that is std. for boats without big dodgers or biminis..New or used, this is the way to shop
Good luck!
Seth Morrell