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