I guess I'm not sure why the "golden number" would apply to reading devices. I get how it applies to pictures...as in, I understand the concept... but I've seen great pics that don't conform to that rule. I think it is more a training aid to get the photographer to think about composition...
I don't see how getting up to 60-66 characters on a line
really makes a difference??
I use my phone to read, exclusively. I typically have it in landscape, but that is so the tables that are often in the books display nicely. It feels like the same width as an old style paperback book - maybe even a little wider. Although, my eyes have reached that 42+ years old range
and I find myself making the font a little larger, I don't notice that there are fewer characters on the line. I just get to the end and my eyes move to the next line. I also use auto-scroll so I don't worry about page flipping/swiping. I don't notice anything until I get to the end of a chapter... unless, of course there is some atrocious grammar/spelling that jars me out of the story.
FWIW, I counted the number of characters in a line and it is about 45-50.