ref: +/- 30 degrees. Years ago in the days of PANS Ops introduction I was told that a circling approach was anywhere the final approach was outside 30 degrees of the RW C.L. Thus it was required to manoeuvre more than 30 degrees on short finals. I can't give a link.
How do your guys advise to fly a visual circuit? That should be a descending base leg onto finals for a CDA. A circling base leg onto finals is also a visual manoeuvre.