I was just at Exotic Lumber early in the week--I love that place!
I was at the boat yesterday to check the camber for sure. Here's how it turned out...
Fore and aft, there is no camber to the cabintop. That helps.
Athwartships, there is camber that is fairly consistent over the cabintop, whether on centerline or further to starboard over the head. Just to recap, Nicro states:
When installing the Day/Night PLUS Vent on a cambered, or curved, surface, it is critical to check to make sure there is no more than 1/2" (12mm) of camber over a 12" (300mm) length of deck surface. Too much camber will distort the unit’s base plate, causing the cover to be raised off the deck.
I'm not exactly sure which way to measure. Is it option A or B (note that in my sample images the curve of the cabintop is very exaggerated--boat is much less)?
With option A, the middle of the 12" ruler is at the center point of where the vent will be, and the space at each end is about 5/16" or so.
With option B, the right end of the 12" ruler is flush, and the space at the left end is about 5/8". The middle of the ruler is still over the center point of where the vent will be.
I'm guessing that Nicro intended option B to be the correct way to measure, but not sure. If anyone has any input on this, let me know.
In the meantime, I'm assuming it's option B that's correct, and therefore I'll need to correct for the camber. Right now, I'm leaning towards a teak block that's square at first, so it's easier to draw the camber lines on the block. Then I can cut/shape the bottom of that block for the camber, and then cut it into a circle and finish it.
That's where I am right now, but I haven't started any of this yet...