When I want to know more about a number, I sometimes seek to test the assumption that an order of magnitude more or less (1.5%, 150%) is well outside the bounds of usefulness—trying to get a sense of what range the number exists within
The 5 Step Design Process emphasizes making requirements "less dumb," deleting unnecessary parts or processes, simplifying and optimizing design, accelerating cycle time, and automating only when necessary.
Musk suggests that if you're not adding requirements back at least 10%-15% of the time, you're not deleting enough initially. The percentage is an estimate initially based on experience, and now for several years based on estimates from manufacturing practice.
SpaceX probably has stricter processes than your average IT shop, then it isn't hard to calculate stats like that. Then when you have the number, you tune it until you are happy, and now that number is your target. They arrived at 15%. This process is no different than test coverage numbers etc, its just a useful tool not arrogance.