Required space for conductors:
a) 1.5 cubic inches per 14 AWG
b) 2 cubic inches per 12 AWG
For a device over one inch deep, deduct from the actual volume of the box: 5 cubic inches per inch of depth of the device. (for a 1.5 inch deep device, deduct 7.5 cubic inches)
That rule is a good example of poor workmanship. If the drafter of that rule worked for me, I would fire him.