Now I'm certainly no expert. So take my post with a grain of salt.
My understanding is that the idea is to start with some rounds that you know are within pressure specs. Fire those rounds in your pistol. Then measure those rounds and use that figure for your load development.
So take some hot factory rounds like Buffalo Bore or Double Tap and fire those in your weapon. Measure that. If your reloads show the same or lower case head expansion then you are within the same pressure range.
Otherwise it seems like the individual chamber of the barrel is going to influence how much expansion you see.