Calculating and Marking Rafter Length

What happens here

To determine the overall rafter length, the presenter uses a tape measure to get the total run from the outside of one beam to the outside of the opposite beam (233.5 inches). This measurement is divided by two (since it's an A-frame roof), and then half the thickness of the ridge is subtracted, resulting in a run of 116 inches for a single rafter. He then uses a construction calculator app on his phone, inputting the run (116 inches) and the roof pitch (4), which provides the rafter length of 122.25 inches. This length is measured from the ridge cut, and a mark is made on the rafter board.