Yes - that was interesting. Trouble is, there is no reliable method of co-relating CUP and PSI over a broad range of calibres. Some ctgs., like the .44 mag, and .45 Colt, .45/70, .450 Marlin and .458 Mag. run identical or very close numbers, CUP and PSI, whereas others, like the larger cased, small bore magnums show vastly different numbers at the same pressure.
: It was indeed a shock some years back, to learn that 'normal' max pressures were in the 63,000 to 65,000 PSI range for modern rounds and some not so modern.