>the question is how big the "engineering design margin" is...but wind loads go as square of wind speed, so 15mph over budget is significant

NASAs human-rated rockets require a structural safety requirement of 40% with the stipulation that you can have lower margins after a lot of testing. I believe the external Shuttle tanks were 25% as I think the 1.4 margin is so difficult to achieve throughout the whole system.

Edit: The industry standard for non-human rated rockets is a margin of 1.25