That may depend on whether the fudge factor is a simple constant or some non-linear function derived from experimental data. It would be fun to talk for an hour with the technical director at SAAMI to learn about how things such as pressure rise rates, breadth of the pressure peak, etc. affect the error in CUP. I can't prove it, but I have a hunch that a corrected tarage table (one based on comparisons to piezo data) could get you to within 15% of true. Unfortunately, I also suspect the correction could vary with things like SD and relative case volume.

With the advent of cheap stain gauge rigs for handloaders, the whole issue seems to be moot, even if it is interesting.