So, I have the same issue that I see other people posting about. After setting the IPD to the setting that gives me the clearest picture, my eyes still feel strained, and feel like I'm looking kind of cross eyed in order to focus.
I started thinking and reading and determined it has to do with the software IPD, and the fact that my eyes are further apart than what the pimax was designed for.
My actual distance IPD is around 62mm using the method explained here:
BUT if I set the P4K to 62mm it was a very blurry out of focus image, i had to set the IPD to 55-57.
I figured this discrepancy of actual to what I had to use to "get by" with the Pimax was what was causing the fatigue, because the lenses are closer together than my eyes.
So here is what I did:
I went to CVS and bought some cheap reading glasses. I picked the prescription that gave me the best focus about 1' away, I just tried on different ones and stared at my finger like a weirdo.
+1.25, 3 pairs for $20, METAL frame (this is important)
I then bent the frame so the lenses angle OUTWARDS.
I put on the glasses and then the headset and went to the IPD settings.
low and behold at 62mm not only is the center perfectly in focus but MORE of the entire scene is completely in focus, with only a little blurriness starting towards the edges, my eyes also felt much more natural and relaxed, instead of having to try and see clearly, I just did.
Also an important tip, when setting the IPD, CLOSE YOUR EYES FOR SEVERAL SECONDS between adjustments. If you keep looking at the green cross hair while you change the setting your brain will keep trying to maintain focus and it will "trick" you into thinking you found the perfect setting because its clear, but its only clear because your brain is compensating and trying to maintain focus, this is another reason you can have a clear image and still get eye strain, its not "naturally" clear, you are having to force it.
Anyway, this worked for me, and its cheap and easy to do, worth a shot if you have the same problem I did.