I just tried this one handed suggestion - first time ever in my 6 years using an iPhone - and I can see why I never thought of it. Extremely awkward and imprecise. I don't get why anyone would do that. Do you have giant hands?

For zooming, I've always used the double-tap method. This generally works fine for zooming in and out on Safari, and it works for zooming in on Maps. I can do a two-finger double tap on Maps to zoom out, but it's still pretty awkward so I'll keep switching to two-handed mode for those (rare) instances.