I used the location where the high speed water pick up was already located on the starboard side, it was easy to get to (I moved the water pick up to the port side). It's about 30' from the transom. Looking at the Airmar web site's mounting instructions, I think you should be real good at 24" or so. Put it in the easiest location....in the flat area between the chines....for you to get to and work on. It hurts to see a 3 3/4" hole saw going through the hull, but when done it's worth it.
http://www.airmartechnology.com/uplo.../17-364-01.pdf