Now you want to know how I came up with that estimate, right. You said the boat is 18’10", but the length you should use is from the center of mass to the tip of the bow. Unless you are very light, compared to the kayak, that center will be about your center. I assumed that, as in most kayaks, you set a little bit forward of half the length of the kayak, so I used 9 feet as the distance. The perimeter of a 9 foot circle is about 678 inches. Which works out to just under 2 inches per degree. So one inch would be about 1/2 degree. That should be close enough for what you are discussing.