Here's how we did the bracket on my brother's 23'er. This is a bit tricky to explain. We measured 29" from the keel to the center of the top engine mounting holes on the bracket. To do this we took a straight 2 x 4 and used a floor jack to hold it against the keel about 3' out the back (there's a little 3-4" wide flat spot to hold it against) and then moved the bracket up and down against the transom until we got to the 29" number squared up from the 2 x 4. His bracket was 27' deep. Once we got that spot to the top of the bracket against the transom we leveled, centered and drilled. I have no idea what that measurement was on the transom, all we cared about was getting the engine mounting holes at 29". If we did it over we would have done 28", his engine is a 25" shaft eTec and his it's bolted on the second hole from the top, only one hole left to move the engine down. This can be done on a large sheet of cardboard to scale using 13 degrees as the transom angle, oh yeah we had the luxury of a forklift.
Oh I forgot.......drilling holes in your transom is fun!!
