Tug,
If you have the "four corners" marked, you can set the trailer to have those marks all level on a level ground. then you can make a marker ruler or trammel type thing using an old lamp stand or Christmas tree stand or jack stand with a vertical pi;e and a horizontal stick or pipe with a marker on it.
Then just move the base around, keeping the vertical upright and trace the distance from the floor to the boat hulll. It will get pretty funky looking, the shape that you will end up with.
Like Joe, I've heard it's best to put the bottom paint 2 inches above the actual water line. On my TomCat, the marina put the bottom paint exactly on the water line and I had a continuous strip of mold just above that. Water, as you know, rarely holds still - the normal state of water is ripples and wavelets.
John