In the Book "The Complete Guide to Mid-Range Glazes" John Britt writes about reading cones, "The perfect arch of self-supporting cones is when it is bent to the top of the flared triangular base". He goes further into the subject with illustrations, photos and lots of great information. If you don't already have John's new book, you need to stop what you're doing and order it right now.
I found this video from Orton. It was very enlightening.