If you have access to the drawings, then measure in several places, level on the bench according to your marks and then run a pencil mark around the hull. Then you have a continuous line to align the tape with.
Depending on the hull build, you might have to measure down from the deck (My dad added .75" to the depth of hull to gain displacement for batteries etc.) This led me to measure from the deck line.
Once model is in the water, shift ballast around until it is level with the line.