Hi Chris, now this is what I would say is a really good question, simply because the instructions don't give any indication nor has there been a definitive answer on it that I can find. All I can say is that on my build I went with measuring on the vertical and for me it worked out..... .. Not the definitive answer you're looking for but the best I can give I'm afraid..... Others may have measured following the profile of the hull but nobody that I can see has said they've done it this way.....

Regards

Alan