Hi George, With all due respect using Pi in segmenting introduces error. The
reason is as you know Pi is a measure of the circumference of a circle.When we
build a segmented ring we are stepping around with straight lines. The segments
have to be slightly longer to acomplish the same diameter. While a ring with a
lot of segments won't have a substantial error a ring with fewer segments will
have more of an error.
Draw a ring with a smaller amount of segments then do the math and you will
find the pi method results in a shorter segment length thus a smaller finished
diameter.
In some designs enough error to cause major problems.



Bob, Naugatuck Ct.
http://www.outofcontrol-woodturning.com