So tell me what I should go by if I can't use "SIGNS"
You do realize how hard calculating chamber pressures is , don't you?
The copper crusher method, Piezo-transducers and strain gauges are the best actual measuring methods we have so far, BUT.....any instrument engineer that's worth his weight will tell you it's a WAG
Even the sophisticated programs like QuickLoad are only an educated guess as there are so many variables and the input has to be exactly correct or "garbage in, garbage out".
This is from a article Terry Hart wrote, (I'm sure you would tell him he doesn't know what he doesn't know ) I will also put a link . It's goes into a lot more detail about chamber pressures
No method of measuring chamber pressure is 100% accurate. If you measure 100 rounds of exactly the same load, in exactly the same gun, with exactly the same test equipment using the crusher method the measurements will typically vary plus or minus 5% from round to round. Some rounds may be off from the average for that group by 10% or more. A small portion of those variations is caused by actual differences from one round to the next, but most of it results from errors, or variations, inherent in the crusher measuring method itself. This is not a perfect science.
You could use the equations for a solid rocket motor theory but you would have to add the calculations for the twist ,heat transfer to bbl & expansion of bbl.
Oh, don't forget the non linear acceleration rate, which a burning gas doesn't have.