Same distance, same text size, and same font?

also, same color scheme? I've seen some studies that show it's harder for people with certain vision issues to focus on text that's white text on a black background than it is for them to focus on black text on a white background, so.. if they're trying to read on a dark theme on their phone, that could be noticeably different from using their E-Ink screen, which is almost certainly using a light theme.

That would definitely be a good experiment to try. I would have to try a light black on light grey to match the kindle.