“But how can you be sure?”
100% certainty is not a variation of 96% or even 99%. It's a totally different category.
Certainty is binary, yes or no. The question, "are you sure it will work" is not about the work, it's about the sure. If you need to know that it's going to work, then you've committed to a very clear path. Some people go to work or school and do nothing except the things that they are sure about.
The other path is to do things that might not work. Work, projects designed to land on the spectrum of not sure.
When someone asks, "Do you have any case studies and rules of thumb from my industry about how someone in precisely the same circumstances did x and got y," it's pretty clear that they seek reassurance and a promise of certainty.
But all the good stuff comes from leaping. From doing the things that might not work.