No need to do diagonal I think, but one do need to use the bitmap size and field of view per eye, because a good portion of the left and right views overlap, to give you stereo vision.
...and since the per-eye FOV is rarely listed, I tend to favour first calculating the vertical ppd, because the vertical FOV is common for both eyes;
In the case of StarVR: 1464/130=ca.11,26ppd
If Pimax's claimed vertical FOV claim can be trusted, and vertical screen utilisation is 100%, their's would be: 1440/120=12ppd
(Note that I used the size of the image actually sent to the HMD, rather than the sceen native, for effective image resolution)
In other words: Much like In the post above your's, where the poster used an assumed 150° horizontal monocular FOV for both the Pimax and StarVR, which is indeed what was claimed by Pimax in the same FAQ I got 120 vertical from. (I'll however observe that in that FAQ, Pimax also state they have about 80% horizontal screen utilisation (haven't heard about 50 before, but maybe that's including the rounded corners..?), so more: 2560*0.8/150=13.6ppd, than 17. (The FOV he used for CV1 was also significantly enough too much larger than real life, that it produces a 20% error)).