Also... Be aware that the curvature of the hull happens above and below the line you are measuring. The thickness of the board, may be enough that the top does not sit flush to the hull, without relieving the underside. Easiest way to make that cut, is on a band saw with the table canted over.
Levels don't work... but bevel gauges and protractors do.
If you like the level of something on the same bulkhead, you can measure what its angle is and get a small step farther than eyeballing it. Basically you have the choice of either building off an imaginary level, or using bulkheads and pre-existing stuff as a level flat surface.
I like to use a block plane to shape while mocking up, but I tend to use plywood
scraps rather than cardboard as a template.
Lastly... You can try to shoot for perfection, (I know I spend way to much time trying to get things perfect...) but remember that anything under the piece of trim at the edge doesn't have to be perfect. If the board is going to have formica, or another hard laminate on it... the laminate can be cut perfect to the hull, and the board can miss by an 1/8th inch here and there.