I needed a new Main for my Bav 36, and because I don't live anywhere near a sail manufacturer I contacted Kemp Sails and then downloaded their measurement form and guidelines. With that each of the measurements required were explained and I did the measurements myself. These were submitted to Kemp's, and they confirmed that my measurements were pretty much the same as their records for a Bav 36. A new sail was duly supplied and it is a joy to behold and to use, so ask Kemp's, or your preferred sailmaker for their measurement form and guidelines, and you can do the measurements yourself. Just remember when sending the end of your tape measure up the mast, to attach a length of strong line or something similar to the topping lift halyard so that you can pull the halyard end back down in the event that the tape measure breaks.