You will also want to read this further explanation of how to do centimeter measurements, correctly: Centimeter How To
How Does It Work?
It simply calculates diopters (diopter being nothing more than inverse meters). The variance is how you look at the 'edge of blur'. Important to note here is that you want to be where things are still clear, but another half centimeter further away, you notice a loss of sharpness. This requires a little bit of experimenting.
You can also look at your current diopter values, and the related centimeters - so now you can get that exact distance to the screen and see how sharp the image is (without wearing glasses). How much can you move backward from your centimeter equivalent of diopters? If you can move back and still see sharply, you're over-"prescribed".
Be sure to keep a log of your measurements. Especially if you are participating in BackTo20/20, you will want to know where you started, and how much you are improving. Read this topic about keeping a log. Also, for a better understanding of why your prescription is quite damaging when you use it for close-up work, take a look at an explanation of the blur horizon.
We discuss the topic of excessive diopters and it's contributing effects to worsening myopia in various parts of the site and as part of vision improvement strategies.
Click the link below to be taken to the centimeter calculator.
Take a moment to read the instructions. It takes a small measure of patience to find your values the first time - quite worth it! You can easily check your diopter value, and the degree of changes throughout the day.