Backboard Pixel Distance Question

[Glen]

The 2012 Vision whitepaper explains on page 9 how to compute the distance from the target. We understand everything but the 56 pixel wide target rectangle (This is the actual quote):

The target width measures 2 ft wide, and in the example images used earlier, the target rectangle measures 56 pixels when the camera resolution is 20x240. This means that the blue rectangle width is 2*320/56 or 11.4 ft. Half of the width is 6.7 ft, and the camera used is the M1011, so the view angle is ~47˚, making Θ equal to 23.5˚.

My question is this, where did they get the target rectangle width in pixels (56 in their case)? After all, that isn't a fixed number, it will change accordingly based on how far/close the robot is to the target.

Some clarification would be great.