The height of the vents should correspond to the bottom of the flap on one's jacket pocket. This means a slit of between seven to nine inches on a size 40 regular. If higher, the vents will simply call attention to themselves.
From Clothes and the Man by Alan Flusser @Fashionmall.com Excellent read. Check it out: Clothes and the Man
I do remember that some British tailors liked to cut the side vents very deep, ending even above where the pockets where. Flusser's advice is sound. Still, I like my side vents starting right above where the curve of my butt starts, which would be right where the pockets are. I think that should be the only constant you'd have to worry about.