That formula is for the least squares straight line fit to of a plot of Maximum Average

*{peak}* Pressure (MAP) in CUP vs psi. It would be a good conversion recipe if the correlation coefficient were higher (very near to 1), but it isn't high enough, for my mone. For example, the .223 Remington and the 270 Winchester both are rated for 52,000 CUP MAP by SAAMI, but when they measured the same reference loads used to cross-calibrate copper crushers in the conformal pressure transducer, the .223 Rem came up 55,000 psi MAP, and the .270 Win came up 65,000 psi MAP. According to the formula, they should both be 61,138 psi. So it is 11% too high for the 223 Rem, and 6% too low for the 270 Win, and that's about as well as you can count on it to do. (See

page 4 of this article where it originated.)

The load could still be on the light side. I just don't have an easy way to prove it without measurements.