That does seem simple, although we once picked up a 70' slip for a 63' boat that measures including all extras 64'2". And the kid comes out to measure. I tried to turn it into a learning experience for him but failed. I asked him, "do you know the size of the slip?" "Yes, it's 70'". Then I said, "well now look at the front and the back of the boat. Aren't they both inside the slip?" "Yes". I then say "Well, then you know without measuring that they are less than 70' and so you charge for 70' don't you?" And here's the answer. "Yes, but I still need to measure." Turns out it was the dockmaster's rule as he just didn't feel he could trust the dockhands to make any judgement on their own.