I can only tell of my one experience, others with more experience may know more about it. I used the same box beam external strong back design (5/8" ply). After assembly and setting it up on 2 horses and supports, I noticed it soon was not perfectly straight. I allowed it too settle (2 weeks) and resupported it. I was able to be then certain it was stable and would not settle any more. Then to my mind as long as it would not change from that point IT WAS FINE TO USE. I just used it as a solid platform and set all the forms straight upon it using only the string line as the guide. Then any "bends" of the strongback had no effect and if it no longer moves it is fine. I did this and my boat is straight as an arrow. Good luck.