Rjp44,
Check this out. It may solve your problems.
http://www.airmartechnology.com/uplo.../17-217-01.pdf
I also seem to remember that the DR for a 23' Scepter is 22' at the keel panel, going to 17 mid and then 11 at the outer most. The transducer above may not be the only one with the variable angle feature. Check around. Also, remember that a boat seldom sits perfectly level in the water. I would bet the best soundings are +/- 3-5%, and probably are more repeatable than actually accurate.