I don't currently have any known vision problem nor wear glasses, I was only looking to reduce eye strain since I regularly need to take breaks due to my eyes hurting after using the computer for too long. I have had recent eye exams and they were 20/20.
That being said I'm even less sure how to calculate it now.
The post at the top of this sub-thread is specifically about myopia. It is not applicable to eyes hurting and any calculation would not be applicable to someone without myopia, since the calculation begins with a myopia distance prescription.
Myopia or no myopia, afaict the way to reduce eye strain is the same.
My understanding is that eye strain is either caused by unbalanced lightning (e.g. bright screen in dark room), or focusing on close distance for too long without giving the muscles a break by focusing further away. The well known 20-20-20 rule tries to establish that. As a practical advice, you may try adding some small walks, getting some fresh air for a few minutes, or even looking out of the window for a few minutes throughout the day.
FYI, iPhones have a feature to tell you to use your phone further away when you use it glued to your face. If you have myopia, it should go without saying that you should never use your phone with your glasses.
> you should never use your phone with your glasses
Apple could detect that the user is wearing glasses. During FaceID registration, the phone could ask whether the glasses are for distance (myopia) or closeup/reading. If distance, the phone could offer to remind the user to remove their glasses after using the phone for more than X minutes.
That being said I'm even less sure how to calculate it now.