Once your boat is on the hard, run plumb
line down from the fartehest point forward, and aft, then measure. That is usually the easiest way. I would also NOT be suprised if your boat is about a foot longer than speced, being as marina's charge by the longest part. Along with my marina, goes from 37'3" to 38'3" is a 38' boat, so if you are 38'4" you're a 39' boat to them!
marty