It's really a matter of distance. Your reading glasses bring things into focus at your reading distance, which is approximately 14 inches from your eyes. Anything outside of this range, like a computer monitor which is further away, will start to appear blurry. The key thing to remember is this: One lens strength will not bring things into focus at both reading distance and computer distance. Well the simplest solution is to use reading glasses with a weaker lens strength. The general rule of thumb is that decreasing your lens strength by 0.75 will bring your monitor into focus. For example, if you wear +2.00 reading glasses, a pair of readers in a +1.25 strength will allow you to clearly see your computer screen.The key point here is: The weaker the lens strength, the further the focal point. The only downside to this simple solution is that with the weaker reading glasses, you now lessen your ability to see things at a closer distance. You may want to play around with a couple of strengths and find a balance between seeing your monitor and your printed material.
Theoretically, reading glasses is for looking the object nearby and the eyeglasses used to look at computer is for looking middle distance. There are several solutions to these differences, besides simply buying separate reading and computer glasses. One is to have special bifocals, with one part of the lens dedicated to reading the printed page and the other part customized for computer screen use. This isn't an ideal situation for all users as, unlike reading a page from top to bottom, it's more likely the eye will need to dart about when using a screen, which can require head movement when using bifocals. A more sophisticated solution is progressive addition lenses that, rather than simply being split into two lens strengths, gradually increase power moving from the top of the lens to the bottom. This allows easier adjustment to different types of visual material.