One thing that he didn't mention is that you'll actually want to take the measurement between the marks and add about 1/2 inch (1.27 cm) to it, especially on a bit like a loose ring snaffle. That way, the bit will be wide enough that it won't pinch the lips or face.
If you buy a bit that is exactly the width of the horse's mouth, then a loose ring snaffle will pinch and sore the corners of the mouth. You could probably get away with it if you're using an eggbutt or a full cheek, but not many other bits would be as forgiving. If you buy a curb bit the exact width, then the purchase rings will actually dig into the sides of the horse's face because their face gets wider just above their lips.
Most horses that comfortably wear a 5" bit will actually have a mouth measurement of ~4.5".