We measure your quality of vision at 20 feet-this is the numerator (top twenty). The denominator (bottom 400) is a comparison to someone with standard vision. So if you have 20/400 vision you would have to be at 20 feet to see what one with standard vision would see at 400 feet.
20/400 does not correlate to your exact prescription, it is merely a measure of your vision without (hopefully) your eyeglasses or contact lenses on.
By Dr. Cook, O.D.