Tim, good question, but not a simple question at all. There is no direct conversion formula to convert psi to LUP, or vice versa. Supposedly, some of the so-called conversion formulas can be far enough off to provide dangerous and unreliable information. The best you can get is approximations that may or may not come close to being accurate.
I've gone down this rabbit hole before, and found much of the same goblety-gook that you are probably finding from internet shotgun experts.
Fortunately, it probably doesn't matter much because virtually all shotgun pressure measurements being taken today rely upon piezo-electric transducers, which react much differently than directly measuring the compression of lead or copper crushers.
