If you have a compass and can find the center of the board, you can draw this directly on the wood rather than glue on a pattern. To calculate the cutting angle, use this equation:
angle = arctan(ring thickness/board thickness)
in this case,
angle = arctan(.375/.750) = arctan(0.50) = 26.6 degrees
If you don't have a calculator to make that arctan calc, try this one:
http://web2.0calc.com/. If you click on "2nd" you'll see along the left edge the word "tan" change to "atan" (same as arctan).
Remember you need to drill the starting holes at that angle as well.