OK that's good information to know. I'm not an expert--don't claim to be--especially with the cartridge you're testing. My point is this--as far as I can tell you are trying to figure out whether the buffalo bore is or is not safe to use in the cartridge that has a very small "pressure tolerance" band between roughly 7,000 and 12,000 psi according to the figures you are using. When you are "correcting" the deviation from what is expected to what is returned by the instrument--it's very important to know what the accuracy of your baseline measurement is. That's why I would call an ammo manufacturer and (being really nice, because they probably have lawyers telling them to be careful for liability reasons) ask them what their pressure baseline is for an off-the-shelf retail cartridge is.
All of the cartridges I have tested for generally are in the range of 17000 to 65000 psi. A deviation of 3 to 4000 psi is not going to be a big deal as long as I know when I'm in the vicinity of the upper end of the safe pressure limits. Your pressure limits I'm guessing are probably dictated by an older cartridge originally made for a black-powder, low pressure tolerance old rifle/revolver. Your weapon's "tolerance" for pressure range is so small that I would be very reluctant to rely on the instrument's output alone unless you had a very high confidence in the pressure rating you use as a baseline for calibration/correction.
Don't tell anyone I said this, but I would be looking at one of the top ammo manufacturers like one that begins with an H and ends with a y to see if they are nice enough to help you with this.
As an aside--it gets really dodgy when using the instrument for testing a new wildcat for obvious reasons.
I also use "predictive" aids like labradar and quick load to see if they get me in ballpark with velocity and pressure ratings reasonably close to what I'm expecting from the pressure trace.
PS--another thing is that the ammo manufacturers can use a powder blend that is made to their specifications so your chances of matching their results for a given velocity and a given pressure with off-the-shelf consumer powders may not be a reasonable expectation. Those clever engineers at Hornady are particular notorious for achieving velocity results at lower pressures that I have been generally been unable to match in my handloads.