most of the mainsail covers i see are made to fit too tightly. Covers should fit very loosely, so the cloth is not tightly creased, and so air can circulate. If you don't have actual measurements, pick a boat with the same P (foot) measurement (or a little longer) , and two or three feet bigger in the P (luff) measurement. The lighter weight the cloth, the smaller the cover can be. Also, closely spaced slugs tend to flake differently, so you can use a smaller cover. For most boats, OEM grade sails (typically of lighter weight, less densely woven dacron) fit into smaller covers. Judy Blumhorst Hyde Sails of Northern California www.judybsails.com judy@judybsails.com cell: 925.208.1692 fax: 925.820.2327 skype: judith.blumhorst www.HydeSails.com/NorthAmerica