This equation allows you to approximate the distance a horse and rider will travel when they accelerate as quickly as possible to the horse's estimated top speed. This is obviously just an equation used to generate a gross estimate and is used purely for conversation purposes.
You may enter your speed estimate for a horse and rider or use the default value which comes from the vCalc constant: Top Speed of a Horse. You may also enter a duration for acceleration to full gallop, or use the default estimate which is based on Penny's acceleration period (Penny is a competition Polo Pony).
This makes a number of assumptions, obviously, as the time will vary widely depending on the type of horse, the rider experience, the maximum speed of the horse, whether the horse had oats or Wheaties for breakfast, etc.