CG for ringmaster S1 ans S6 vary from 1.25" to 1.5" from leading edge
Sterling Models produced and published a small booklet "Secrets of Model Airplane Building". Originally published in the 1950s, it was included in K Series kits up through the 1980s.
This booklet was expertly scanned into a pdf file by this site's major contributor, David Braun and is now available for viewing by selecting the link below.
Link to Secrets.pdf