OK, well, I guess the nature of my question is: is that sizing universal for all altimeters, or are there specific sizing requirements for the JL products?
It would be universal. All barometric altimeters work the same way, they use a barometric pressure sensing IC to determine the altitude. The venting just ensures that the air pressure inside the bay can match the pressure outside so that an accurate reading can be taken, if you don't have sufficient venting the air can't enter/exit fast enough to keep up with the relative pressure changes and you can get bad readings. You do need to ensure the sensor isn't close to the hole, as the air moving in/out as well as cross-winds could affect the sensor. This is probably less of an issue on the A2/A3 since they're enclosed units (I'm not even sure exactly where the air is meant to flow through the case to get to the sensor, it's clearly not air-tight so I guess it's just through the gaps/seams), so it's harder to blow directly on the sensor. On units where the sensor is exposed they tend not to like direct sunlight either. So the farther the sensor is from the holes the less disturbance (and light) it should see. And the number of holes (as I understand it) is to have an even distribution around the airframe so that cross-winds don't result in a pressure increase/decrease in the bay, but these winds can simply blow through the bay. If you put all 3 holes right next to each other you'd be defeating the purpose of having multiple holes.
If I understand the OP's post it sounds like the A3 is riding in a parachute section of the rocket, so it means more/bigger holes in a section meant to be separated with BP. I'll note that these sensors also don't like being exposed to burning stuff, again the A2/A3's enclosure should help here, but I'd definitely make sure you protect the A3 from the charge as well (put it on the opposite side of the bay from the charge, or along with the chute protector or with its own protector, etc). I think in theory even sudden severe pressure changes can be risky (there's usually a small membrane in these sensors and if you rupture that it won't work right, think of it like blowing out an ear drum), though I'm not sure how a BP charge compares to what they can safely withstand. You will very likely see a spike in the A3's reading when your charge fires (see my example below where I flew my A3 in the parachute section of my Nike Smoke, with a single vent hole, you can definitely see when the motor eject fired), and you do have to strike the balance between sufficient venting for the barometric sensor and too much venting preventing the charge from producing enough pressure to separate your rocket. Definitely ground test, shouldn't be any need to have the A3 in the airframe when you do the test (why risk damaging it on the ground).
Ultimately, I would argue that since the A3 isn't used for deployment (only logging), the accuracy of its reading isn't quite as critical as a deployment altimeter (where a confused reading could result in early or late deployment). So personally I favor the low-side when using my A2/A3 in charge-equipped sections (sizing the hole just to prevent pressure separation during ascent, not to be optional for barometric readings) to ensure there's still sufficient pressure for separation when the charge fires. By the time you reach apogee the pressure isn't changing as fast as the powered ascent portion, so the airframe will have time to equalize pressure and you should get an accurate reading of apogee, you just may get a slower reported ascent (more linear and less of a parabola) if the air takes longer to get out of the tube. But if too much of your BP gasses rush out the vent holes and your rocket doesn't separate you're going to have a bad day. I'll take a less accurate flight log over a lawn dart any day.
BTW, I don't understand why the A3 saw a decrease in the altitude around burnout, maybe that was due to the contents of the rocket shifting around or something?
