This handmade sheet was unearthed in the Cinerama archives by producer-director David Strohmaier during his research for the production Cinerama Adventure (See our links page to visit the Cinerama Adventure website). The sketch has been reproduced in a number of publications such as American Cinematographer and The American Cinematographer Manual. It's as good a source for bar bets as you're apt to find.
NOTE: There are numerous websites and other resources that will tell you that Cinerama had an aspect ratio of up to 3:1. When you find such a source it's probably a good idea to quit reading at that point because the author has done absolutely no research on the subject. As is seen here, the camera aspect ratio is 2.59:1. The optimum screen image, with no architectural constraints, was about 2.65:1, with the extreme top and bottom cropped slightly to hide anomalies.