I am sure there are any number of aerodynamic experts that could give you a much more technical answer. However, here are my 2 cents worth of accumulated wisdom:
Just flatten the heck out of your mainsail or
jib by maximally tensioning the luff and the foot (with a light breeze blowing across). If your vessel is set up with a backstay adjuster, also optimize mast bend for maximum flattening.
If the resulting "blade shape" does not look like something you could fairly well approximate by simply bending and twisting a thin metal sheet, I would either have the sail recut or replaced.
However, if you are more of a cruiser instead of a racer, don''t worry about any of this until you start burying the rail regularly at less than 50% of maximum hull speed.
Have fun!
Flying Dutchman