Say you want a two inch stripe on the waterline. If you were to float the boat in water and mask the hull on the water line, then sink the boat exactly two inches and mask again, the only way the strip would measure 2" is if the hull side is 90 degrees to the water. The more flair in the hull the wider the stripe. The result in a boat hull would be the appearance of the stripe bending up like a banana. The only time it would look 2" is if you were to sight down it from one end. This is what happens with a laser level.
If you were to take the first masking line, done exactly on the water line, then measure on the side of the hull up 2" all the way around, the stipe would appear to bend down in the ends when viewed from above.
The only way to get it looking just right from the angle the stripe is going to be viewed at is by eyeball. Done wrong and it will look odd but hard to explain why.