If you are looking for a flush (internal or thru hull) ducer, finding the best location for it may have to come first before you can figure the beam angle offset.
Hull angles vary from for to aft & from keel to chine.
madhatter1 is correct on a fairing block if installing the better thru hull ducers. Again, find the best location, then find the angle of the hull for that location, transfer that angle to the fairing blocks cut.
Many of the better thru hull ducers come with or have an optional block. These and the included instructions do help the install greatly.