Calculating Add for computer work

I made it clear to my optometrist that I would probably get less strain if I wore glasses for computer work. So he gave me what now turns out to be a reading prescription. It didn't dawn on me at the time when he said "hold this book in front of you while I put this in front of your lens", that they would be no good for computer work (unless I get much closer to the screen)

My Add is +1.00 so is there a rule for working this out for computer work with a screen that's about 70cm away?
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Mike News
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