I looked through an Alliant reloading manual for that load, and with 20/28, Unique, Herco & Blue Dot the pressures ranged 9000 - about 10,000 psi.
IMHO it is very unlikely that non-specialty loads (or B&P) would be LESS than 9000 psi.
Was not aware that the Herter's tradename is now owned by Bass Pro-Cabelas and the shotshells are by Winchester; which of course may change as can components & powder
Some time ago I contacted Remington, Winchester & Federal regarding 16g pressures and no one responded. The only way to have factory loads tested today is by Tom Armbrust.
re: SAAMI recommended standards, but NOT carrying the force of law like CIP standards.
SAAMI allows the maximum average pressure for 12 gauge 2 3/4” and 3” shotgun shells to be 11,500 psi +/- 900 psi, with the maximum extreme variability not to exceed an uber-max of 12,500 psi +/- 900 psi or 13,400 psi.
CIP 12g 2 3/4" Maximum Statistical Individual Pressure is 850 BAR = 12,328 psi
