I always wondered how 645 people measure the distance since they don't display that numbering.
Usually with two windows using the 6x9 numbers twice. i.e. Frame 1 is with No. 1 in the first window, frame 2 with No. 1 in the second window, frame 3 with No. 2 in the first window... and so on until you get to frame 16 with No. 8 in the second window.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.