Although this is not that interesting for Pimax 4k users (since the Pimax 4k IPD is not your real IPD), there are several people here with other HMD's, who need to know their exact IPD. So as I needed to enter my exact IPD in my Vive Pro I tried measuring it using the supplied method, looking into a mirror with a measurement strip and trying to measure what my IPD is. This works pretty well but it might be hard to get to the exact mm. So I thought of a new way of doing it and it worked suprisingly well:
First measure your IPD with the above described, regular method. Let's say you measure 65mm so you know that's ROUGHLY correct. Now we're going to get it more exact:
1) Take a piece of paper and mark two dots, your IPD distance apart, so in this example 65 mm between the center of the dots.
2) Now punch the dots with something sharp, so you get about 1 mm holes, I used a nail cutting scissor, works really well, slowly turning the scissor so that you can make a nice round hole.
3) Now bring the paper to touch your nose. Slightly rotate the paper clockwise, so that the hole for your left eye is a bit higher.
4) Now look through that left, higher hole and focus to some point in the distance. Relax.
5) Now keep looking through that higher hole, while focusing on the distance, slowy rotate the paper against the clock, so that the hole for your right eye gets closer to the hole you're focusing on. Keep focusing on that left hole, and on the distance!
6) So from your peripheral vision you see the hole for your right eye coming up higher, closer to the left hole. Now if you see this hole passing to the right of the hole you were focusing on (the one for your left eye), then your IPD is LOWER than the distance between the holes! Similarly, if you see in your peripheral vision the hole that's rising up passing to the left of the hole you were focusing on, then your IPD is higher then the distance between the 2 holes.
It's important that you keep relaxing your eye muscle and keep looking through that left hole, so don't start to focus on the right hole, only see it from your peripheral vision.
So you can then do the experiment again with adjusted distance (so punch 2 new holes at a different distance) and keep doing it until the 2 holes are perfectly aligned.
If you do this correctly you can get it down to about 2/10th of a millimeter I think, maybe even to 1/10th correctly. For example if you do this for 65 mm and for 66 mm and find your IPD to be between these, you can see which one is closer to the middle. If that's the 65mm one than you know that it's somewhere between 65.1 mm and 65.4 mm and can get a reading that I guess is pretty much as specific as a professional one.