If you are having a new main
made, you should measure the three sides of the sail area you want to fit yourself using any number of procedures/forms available at most of the online sailmakers websites. Or have the loft making the sail take the measurements. I would not recommend using any off-the-shelf manufacturer's measurements, particularly for a main. Also, most lofts will come to your boat to custom measure your boat and discuss your needs. This costs more than some of the offshore sailmakers, but may be worth it. When I had my new main main by North, they came to my boat to measure and we decided to reduce the leech measurement a few inches in order to accommodate and clear my bimini. It was great service and well worth the extra cost as the sail fits perfectly.