HI John
Okay did my best
My epubs were converted from sony.lrf files that had a font size of 10.
I ran them through Calibre with a font size of 0 (so it uses the sony default sizes), line spacing 19, as I like deeper spacing, and margins of 8. I use the font Grandesign Neue. All this should help, I hope......as you will be able to get more characters by using smaller margins and more lines by putting them closer than I use.
The Sony uses Small, as the default font size, it has XS, S, M, L, XL and XXL sizes.
I never use landscape so it surprised me to discover that it doesn't reflow the text in the same font size, but stretches the portrait view over the landscape mode, so the font becomes larger in the same size than when in portrait. Probably quite usual for ereaders but I never knew that till now
So I would personally have set the font to a size smaller than I use to give a similar font size when in landscape. But I have done the readings in the sizes you requested.
Okay, in landscape mode I get the following:
Medium= 34 characters and 10 lines of text
Large= 26 characters and 7 lines of text
remember I used deeper line spacing, as I don't like words cramped together, most people would have them much closer than I do. I guess it would not be that much different to what you have now.......
You can choose which way the orientation flips, to the left or right, so you can have the button of either side as you prefer. On the left side the page turns are nearer the top, on the right they are nearer the bottom, are easy to use in either position, but are possibly a bit better on the left as they are higher and fall under the thumb nicer.
Hope that helps