I have a slab reefed main on my B36(2002), but went to the Kemp Sails website where I was able to print off their measurement guide. Armed with that, a length of cord to be used for measurements and a tape measure to measure the cord and a pair of binoculars, I was able to get all of the measurements I needed from the top of the cabin roof. Hoisting the cord up the mast to the outlet from the top sheave enabled me to measure the leech and the luff, while measuring the foot could be done along the boom. All other measurements were either done with the tape measure where these were to hand, or by estimation, using the fore and aft dimension of the mast as a guide. When the filled out form was submitted to Kemps, they checked it against their records for that type of boat, came back with the comment that my measurements tallied closely with their records, and they then got on with the job of making me a new main sail which fitted perfectly.
Surely you could do the same, but don't send the end of the tape measure up the mast without something else attached to the halyard to bring the end back down if the tape measure falls apart. Be warned !!