243,
I only rely on QuickLoad 3.5 for parametric studies. Their absolute values, for the most part, have never agreed with my experience over the chronograph. Part of their error has to do with making educated guesses about bullet friction in the bore, start pressures, burn rates, things like that.
To be conservative I adjusted start pressure down to about 1800 psi for the data I posted. It may be even lower for the rounds the OP was using because of the apparent neck tension problem.
Anyway, I am not sure I understand your last query exactly. The HPT round generates 70,000 psi. I think those are used for checking integrity of the chamber/bolt/barrel structure (proof load). I may be wrong but the back of that cartridge does not fall apart, probably because it is designed not to at some pressure even above 70,000 psi.
If I keep all other parameters the same that I used to do the initial calc.s, 28 gns of Benchmark, and just vary the COAL, QuickLoad says:
2.200 61,000
2.150 63,000
2.100 65,000
2.050 67,000
2.000 70,000