Lay your CP-1 on a chart, and align the center of the circle and the center number along a line of longitude (one that goes from North to South). The bottom edge of the plotter now lies along a line that runs East to West (true), or 090/270 degrees.
Rotate the plotter 10 degrees to the right (clockwise). The bottom edge now runs along a line that is 100/280 degrees. What number appears on the top index?
You can do the same thing along lines of latitude (East/West), but that method is less accurate due to the fact that such lines are not straight, they're curved. When you use them, the numbers on the index will center around 360/180.
Does that help?
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