HN user
You're right, when the server receives two swipe events at the same time in opposite directions we assume that both devices are aligned at this point. It doesn't matter though where on the screen you pinch them together,.
We haven't figured out a way to do this automatically. Currently we prompt the user to enter the size of the device when the app is opened for the first time.
I think this blog post is pointless. Of course every abstraction adds some overhead at first. If your problem is simple enough to fit in a few lines of code you don't have to think a lot how you structure it.
Can you please change the github links to not open in a new popup.
I don't think QR-codes wouldn't be an improvement at all. They are ugly and akward. Instead consider printing the URL on the back as well.