If it helps I’m in the same boat using WebView and have spent a few weeks tweaking the responsiveness of my app.
I started with a 380px design width but after finding out that when iPhone 6, 7 are in the Display Zoom mode (something the end user controls), I changed mine to make it 320px wide by 568px high. Though that’s the dimensions for the main page, some groups are actually bigger and it works out okay because smaller phones cut those off and then bigger phones it displays what is intended.
The iPhone 6 I use on a daily bases was set to Display Zoom mode without my knowledge (it’s a setting under display preferences usually set up when the phone is set up) so I assume it’s the case for millions of other iPhone 6 and 7 users to be in this mode.
Overall I’m happy with it, but it was a huge pain in the @$$ to make it responsive. @mvandervoort did you run into issues of having the app zoomed in when loading on an iPhone 5 or 6, 6s, 7 when Display Zoom is turned on? Wondering if it’s possible to have it set up in such a way it’s not an issue being zoomed in from the start, bc that would be ideal.