[JohnMGD]

This is what I found online:
A good, simple way to make a template for cambering deck beams is: 1. Draw a straight line on a piece of paper. 2. Make two marks on it, representing the maximum beam of the ship. 3. Bisect that distance, and mark the center. 4. Figure out the distance between the height of the deck at the ship's side and the height of the deck at the centerline (i.e., 1/4" for every foot of the ship's maximum beam). Make a mark at the center of your line that distance away from it. 5. Take a thin piece of reasonably flexible wood (plastic strip would work, too), lay it on the paper, and flex it so that it touches all three points. 6. While holding both ends and the middle of the wood strip in position, trace along the edge of it with a pencil. You now have a template for every beam in the ship. Just be sure to keep the center point lined up.

This is, (the red text) of course, for a real ship, to transfer this for a model just divide the measurements by the scale !

So for "Scharnhorst" this would mean (0,25 inch=0,635 centimetres), 98 ft. (30 m) X 0,6 cm= 5,88 cm (588 mm) divided by scale 1:150 = 4 mm deck camber midships.
The deckhouses etc. you can adapt to this camber by making a template as described above.