Fly Proportions: Dry Fly
As with all fly styles, there are artistic variations on these proportions listed. This is only for the experienced tyer, since there are still ratios that have to be maintained for a uniform look.
Also keep in mind that some flies when tied according to the original formulas differ from these ratios, usually since they represent a particular insect or food item that varies from the typical one. If in doubt, get a book on tying patterns. My personal favorite is Leiser's book.