Hi Rey,

here's what I did with my 22 footer.

I taped a laser pointer to a spirit level and placed it pointing down
(alongside) the hull about 3 feet away. You need to get this as square to
the hull as possible. I used that as my datum. Then I built a simple
crucifix-like jig and taped rulers to both the verticle and horizontal arms.
The horizontal arm needs to be able to slide up against the hull and down
the vertical leg. The jig needs to be about the height of and half-width of
the hull. I also taped spirit levels to both arms. Basically, you move
your crucifix down the hull reading the height of the horizontal arm from
the red dot and the distance of the hull from the vertical leg (via the
horizontal arm). Sounds simple but you really need to take your time to set
everything up just so or you may find that your offsets are totally
undrawable! A level floor really helps to simplify the math later on.

Cheers,
arnold