Guide:Reducing normalized
In general, you only want to reduce 0.25 diopters from a full, 20/20 prescription. The 20/20 standard indicates that you can consistently read the 20 foot or 6 meter line (half or better correct) on the Snellen chart from that distance in reasonably good indoor lighting without active focus. If your set up only allows for a 10 foot or 3 meter chart keep in mind that this isn't quite as accurate, you can account for this by increasing your standard to a fairly clear 100 percent of the 20/20 line.
You would do well to wait for the non dominant eye to be at this standard as well, (see ocular dominance). Getting 20/20 once is not necessarily an indication you are ready to reduce, it could just be a good day. Reducing too early leads to unnecessary struggle with excessive blur and eye strain that might slow or even stop your progress. With that consideration, it is a good idea to make sure the 20/20 holds for at least a week if not 2 before reducing.
If you're experiencing any double vision, it's recommended to wait until it clears up before reducing.
When introducing a normalized reduction you should perform a "zero diopter reset". It is a good idea to keep the full correction (your previous normz) handy for night driving; at least for the first stretch of time in your new reduction.