Hi. It's Oscar here, the developer of this tool. First of all, thanks for such a great feedback. It's good to know what works and what doesn't
The spirit of this tool came while writing the Vive Pro review for the community. We felt we really needed a common virtual place between all the users and all the headsets out there. Unity or Unreal was the first and obvious option, but sadly, I didn't have the time to develop the app for all the headsets SDKs at the same time I wrote the review. So... SteamVR was the quickest idea. Yep, it isn't the best option, I know, but it allows to share the "destination" between users out of the box. It also has all the OpenVR stuff inside, hence all the support for all the PC VR right now (A Vive, an Oculus and a WMR sharing the same room at the same time was too cool to let it pass)
About your ideas. I'm 100% with you. A slider for the horizontal FOV could be great, i.e. But the limitations of this whole idea are still there: as we are not robots, there's no way to do this in a purely mathematical way. This app is just an approximation of the FOV values, Glare aberrations and a very personal feeling about what it's readable and what's not. For me, as a reviewer of all the headsets on the market, it gives me a point of reference, not an exact value.
What you say about the eyes and the physiognomy of the human face, it's true. That's why I think this kind of approachment to the problem is good enough to let me compare the experiences.
If you want to try the vertical FOV as I had in mind when I design it: place your eyes on the first black bar until it disappears. That's the 0 point. Then, look to the 0º degree mark in front of you keeping the last bar hiding behind the middle bar. Doing that we can measure -again, more or less- the angle from the same eyes position.
The Vive Pro gave me around 90-95 vertical FOV with the official's face plate. So yes, I found that all of the headsets are less than expected. (Rift, Vive, and Pro, so far).
For my reviews, I will write the results with the Spray tool of SteamVR before taking a snapshot. It's a funny way to tell the people that you can write in the air inside VR. (Or I could add a chalkboard for this purpose)
I truly thank you for the time you put to test the app (or "destination") and I'll try to upgrade it as soon as I discover how to do some stuff on that little nightmare that is Hammer (the editor inside SteamVR). For the Standalone headsets and those not compatible with SteamVR, I'll do a unity app. But I'm still thinking about the awesome social mood of the Destinations workshop and the chance to talk at real time with people who have different headsets, comparing them as if we were outside in the real world.