Find a light piece of cardboard and with the blades open, push the head through it. Take a compass find the exact middle of the hole and draw a circle around the cuts making sure you are intersecting the very end of each of the three cuts. Now take a measurement and you will see the listed 1 1/2" cut.

All that being said, keep in mind that this measurement is without stress on the blades. With most mechanicals there is some flex in the blades when stressed so it is debatable whether one actually achieves the listed cut in hard stuff like ribs. Often times the skin will show a larger hole because of the stretch factor.