The ball speed calculation is an approx calculation made using the time between the ball passing the ball sensor and hitting the tile sensor.
The calculation is based on an approximate ball-shooter speed of 12mph.
The speed and angle of the ball shooter, and the return tile will all affect the time and the calculation
The calculated speed is an approximation that should give an indication of the ball speed as well as a general idea whether you are getting faster or slower.
If you require an exact speed then you will need to either use a radar speed gun or do a video camera frame by frame analysis.