You can buy 1+ add reading glasses at the dollar store. They can help with those eye strain issues. It can also work to read some text then close your eyes and repeat the text to yourself. This will help with retention. It is amazing how closing your eyes can help you cognitively process what you just read. Same goes for lectures if they do not require reading a black/white board.

I would suspect that with careful observation, you will find signs that you are reaching your limit of eye fatigue. Any need to reread a line of text has always been my indicator. If I stop reading and move to something else for even a moment, I can go back to reading. Those short breaks at the first minimal sign of fatigue are very beneficial. The opto uses the 2 pages as a preset fatigue limit but for me, 2 pages would have been too much during the early stage.