Case length growth-actually stretching-varies widely according to how much the brass is stretched during firing and the following resizing. And a bunch of other factors. As a result all the trimming instructions I've ever read are to trim AFTER resizing as that's the condition it'll be in when you go to seat the bullet. Right out of the forming dies, there's going to be some variation in case length, especially of straight wall rimmed cases where the headspace is controlled by the rim. Frankly, I've got some .44 brass that's near the 50 year mark and the one time I tried trimming, very few cases needed it.

FWIW, I measured some new, never loaded/fired .308 match brass and found each example was either at or near the listed trim length of 2.05 in. Out of curiousity I ran a couple of cases through a sizing die. The case body didn't seem to touch the die, the neck did. One pass through the sizing die and expanding the neck grew the cases almost exactly 0.005 inches. But that's a bottleneck catridge.

If you just gotta do something, I'd suggest running the cases through the sizing die and set your trim length at the specified 1.275 in. What needs trimmed gets trimmed, what doesn't ignore. Methinks you're over thinking this. A handgun isn't a bench rest rifle.