Getting the leech points correct when building a mainsail can be quite challenging.
Thomas Dreyer(GER) emailed me with this drawing+text about how to get the geometry correct when you are building an IOM (or M) mainsail.
Lester Gilbert has a similar kind of drawing at his site: "Measuring an IOM Mainsail"
Text is currently only on German, if anyone more fluent than me in German wants to send in an English translation then that would be great ! (Or someone who is good at programming could make a program that calculates the shape...)