Note that the elapse time is the approximate time your vehicle should be running. Although many factors influence elapse time the main factor is traction. Usually if your vehicle does not have enough traction your elpase time will be higher than the estimated. The other extreme is also possible and usually occurs when you are using racing slick tires.

The horsepower calculation is correct by the definition of horsepower. Horsepower is a measure of the force required to accelerate a weight to a known speed in a know distance. So the generate horsepower by this calculator is the actual horsepower at the rear wheels.