To calculate the height of the letters, use the Radian angle measurement system.
Thus:
5 minutes-of-angle = 0.001455 Radians
Then:
12 – 20 = 240 inches
240 – 0.001455 = 0.35 inches at 20 feet
So we should say 20/20/0.35 inches to be clear.
Obviously you can obtain the 20/40-sized letter by multiplying 0.35 – 2 = 0.70 inches. The same holds for 20/80-sized characters.
Using these calculations, you can set up a chart for 10 feet. You can easily calculate the size of the letters and cut them out of the newspaper.