ORIGINAL: Crabstick
IMO as far as you can get it within reason, i run my 30 inch mono on 8xl and 16 cells and run the prop about 3 inches off the transom, i would like it to be 4 inches, but its the way it worked out with the speedmaster strut and prather brackets i had.. Ive found monos will plane a lot easier with the prop further away from the transom, in cleaner water so to speak. if the prop is too close to the transom they sometimes tend to take a long time to get on plane and they fall off the plane easy too
My aquajet extreme with 700sc motor and 16 cells designed by offshore electrics had the prop about 1.5" from the transom to the front side of the prop. It took about 6 to 8 feet to get on plane. Not terrible unless you are racing.