The first thing you do with Quickload is enter the correct case capacity of a fired case from your rifle. Quickload defaults to the lowest case capacity and as a example with .223/5.56 cases it can cause a 5,000 psi difference in chamber pressure.

The second thing you do "IF" you have a chronograph is change the burn rate in Quickload until the velocity matches the chronograph.

After doing the above the Quickload chamber pressure "estimates" will be closer to actual chamber pressure.

There are simply too many variables in reloading components and firearms for accurate Quickload readings unless it is tweaked and adjusted.