So I have 2 pieces of oak, 3" x 1" x 10' for the Carlings. (good luck finding that on Google without coming up with larger, or sports - try "Carling and Deck)
All I have to do it cut them to fit.
At each end, there's a compound bevel. And the wood curves along the length of the boat. So if you don't take account of the curve, it will end up short.
You can see here how you cut the angle to get the length right.
I was accurate to about 1/10" over 9 foot on one side, and a snug fit on the other.
Measuring and marking the wood in place, if you can, it easily the most accurate, and least error prone way of doing things.