Gunnutfn49,
The distance between the bullet base and the bottom of the case affects the volume of the powder starts burning in. If you shorten the case necks, but keep the finished cartridges the same overall length, the pressure won't be affected significantly by the shortened neck. The only reason to keep trimming exactly within the SAAMI standard is if you are crimping the cases, in which case the accuracy of the length is for repeatability of the crimp force is important.
I don't expect your LC cases to have enough volume difference among them to matter for your purposes. Assuming the exterior dimensions were identical (usually close enough to true if the headstamps match), the internal capacity of the cases may be approximated by weight. Figure that it takes about 1.5 grains different in brass case weight for the volume change inside the case to equal the influence of 0.1 grain change in powder charge weight. The exact number will vary some with the powder and bullet choices, but that's a pretty good generalization.
A reasonable practice for accuracy loads or for loads that run right up at maximum pressure is to segregate cases with the same headstamp into groups that span no more than three grains weight difference. That makes the influence of their difference no greater than the limit imposed by the precision of standard powder scales (±0.1 grains). For plinking loads, except for obvious damage, if your headstamps are the same, you don't need to weigh cases. For mixed brass for plinking, though, you can find up to around 30 grains difference in case weight between brands. That's enough to matter. It's effect will be about like a span of two grains of powder charge difference. I would sort mixed headstamp cases into groups with not more than 10 grain spans, and not use them for loads all the way up at maximum pressure.
As to the length of the shoulder from the case head, for plinking loads just use a
case gage like the Wilson type, and if the case is within its limits, you'll be fine. Setting a shoulder back so much that it's too short in a gage can allow the cartridge to headspace on the extractor rather than the shoulder. That reduces accuracy and adds to stretching that shortens case life. For accuracy loads in a self-loader you usually want to determine how long the cases come out of the chamber then adjust the seating die to set the shoulder back 0.002"-0.003".
But as to peak pressure, in any cartridge with a pressure peak over around 30,000 psi, the resizing differences among otherwise identical cases make no practical difference. The peak pressure is determined by the volume of the case
after it expands to fill the chamber, so the size of your chamber determines that.