?? X Dimension

[chumbucket]

I've never heard of needing to know the distance from the center of the bottom of the boat to the center of the prop shaft. I have heard of the "X" dimension though. I think MJ is on the right track. What I thought it was (and I may be wrong), is the dimension from the top of the transom (at the point of the motor mounting) to the lowest point on the keel. The motor needs to be set at the correct height which generally puts the cavitation plate at approximately 1-1 1/2" below the keel.
You may want to ask the question on The Hull Truth site or iboats.com to see what they can offer for assistance.
Otherwise, you'll have to ask him to clarify.