The regulations (in the US, anyway) don’t require the use of reverse thrust to calculate landing distances or accelerate-stop distances, but the rules don’t prohibit that use either. What they do say, in both situations, is that another “means” may be used other than wheel brakes provided whatever is used is safe and reliable; is used so that consistent results can be expected in service; and is such that exceptional piloting skill is not required to control the airplane.
In fact, you might recall the Southwest accident at Midway airport some time back? At that time SWA used an “on-board” calculator (some said it was a laptop computer that the F/O lugged around) to calculate the landing distance required using reverse thrust – meaning that it had to work. I’m under the impression that they don’t use that same procedure anymore.