How much error using Pressure Trace even with a reference cartridge can you have with all the variations in firearms?

It seems just like using Quickload and adjusting the burn rate until it matches your chronograph, but how close is it really.

In the Lyman manuals you will see a universal receiver and a pressure test barrel used at minimum SAAMI dimensions. And if a actual firearm is used then they are tested using a strain gauge.

Just how accurate are the pressure readings when using strain gauges vs a direct pressure reading from a transducer.

And does this have anything to do with the swing toward more conservative loading data in manuals. Meaning compensating for possible pressure reading errors and variations in firearms.

Signed
Quickload-MagnetoSpeed user.

P.S. And after MrFurious last posting can we trust him anymore with load data information.