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4 and 6 are the ones I've heard all my life. Don't use it too often and don't look too close. Both are a strain on your eyes.
8 is a very good idea but it also means buying additional things. So if you have the money you might just go for it but the best ideas are still 4 and 6. Don't look at the phone too close and don't look at the phone too long. There's a wide world out there. Don't just live in your screen.