The ratio should be 8 to 1. A longer joint will be stronger -- stronger than the wood. Lessons learned: The scarph joints should be "rough cut" with the table saw before sanding on the vertical belt sander, saving a lot of time and sandpaper wear. The glue did not "expand" as much as expected, but the joint seems to be a good one. It will be tested it to failure.

We'll probably build something like this to cut the scarphs. (We did make something similar -- and it worked great. Lesson learned -- This simple little tool let us buy the cheapest white wood and quickly make clear, long pieces for use on the boat. If we had known this worked so well, we would have saved some money.)