I found some other stuff on the subject of ring sizes here

http://www.xmission.com/~daina/tips/pub/tip0117.html

it is the chart reproduced below for finding the size ring for a given post setup.  Useful if the rings are missing or the manual doesn't list them. WMS manuals don't seem to list them - grrr.

To use it get a string to measure the path the ring takes, look this up on the vertical axis, go across to the red line and drop down to the horizontal axis to find the required ring size.