Side vent jacket question

Do you mean how wide the jacket should be, or how wide the back section is,...? Just some clarification.

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.