Chris in VA makes an important point: the pressure that will flatten a large pistol primer can be enough to damage a gun that is intended to fire cartriges that do not exceed about 20,000 psi.
So, primers are NOT a good pressure indicator for maximum charges in the .45 ACP.
One indicator for auto-loaders is how the brass is ejected. If it is getting thrown farther than similar factory loads, then the pressure is probably higher. But, that is not a sure thing either, especially if you are comparing full power factory loads (which may use a powder with a moderate buring rate) to some of your target loads that use a fast powder.
QuickLOAD calculations are a pretty good way of determining what a change in over-all length does to the pressure when you are starting from pressure-tested data.
To do a QuickLOAD calc, we will need your bullet weight and length, YOUR cartridge overall length, the OAL specified with the data you used, and the powder name and charge weight. If you are using a different bullet than was used to shoot the data you are using, then we will need the length of that bullet as well, although we may be able to find that in the QuickLOAD database if you tell us what bullet it is. Of course, I am assuming that you are at least using data for the same weight bullet as the bullet that you are loading.