: The strongback for my Petrel should be 12 feet long.

I am building the 2 X 4 internal box beam. I count 10, 1 ft spaces between the forms of the 2 X 4 strongback. One is really 11 1/2 inches, but I rounded up. If I count the tapered sections that are inserted into the strongback, then it would be an additional 2 ft for a total of 12 ft. I want to make sure I make the rectangular box beam long enough. I know that section is about 10 ft, but I want to know if it should be a smidgen longer to account for the width of a form and possibly a little more to make sure they do not fall off the box beam easily.