That is a fun question. Boeing had two groups - the design group, and the stress group. The design group does the design, the stress group checks it. I was in the former.

One day, I got called into the stress group, where they told me they were unhappy with my parts. I asked why, they said there just barely strong enough for the ultimate load, only about 1% over. They said they'd feel more comfortable with 10% extra strength.

I said this was no coincidence, I used math to size the part to the load, rather than guessing at a size and checking to see if it was strong enough. The requirement was the ultimate load, which is 150% of the maximum load that could ever be expected.

I asserted that I designed to the design requirement, and an extra 10% would make it overweight. Brat that I was, I said if you guys were unhappy with the design requirement, increase it. They grudgingly signed off on it.

The reason it didn't break on the test stand is because all parts vary in size due to manufacturing tolerances. It was sized to pass the ultimate load test under the most adverse size allowed under the tolerances. Odds are, it'll be a little stronger.

For comparison, spacecraft have (I think) an ultimate load of 110% of the max load. The margins are awfully thin, but they have little choice. If you really want a math heavy engineering job, design spacecraft.

BTW, the stress group signed off on the jackscrew size. If it had failed the ultimate load test, both them and I would have had a black eye. As it turned out, the test guys were embarrassed by their bent rig :-) and fortunately that wasn't really a problem.