I have pretty much figured out all there is to know about unperforated 70mm, but I'm having a hard time figuring out the procedures with perforated film with Mamiya RB backs. First off, what is the perf used for? Am I correct in that it is only used for spacing purposes and that it only needs perforations across the top half of the film for this purpose? I know they don't sell perforated film in cassettes anymore, you have to roll your own, but I noticed that a cassette I got was loaded by Kodak, and it had words hole-punched into it with the film type, emulsion number and code. This got me thinking, how many frames on the front, or inches if you prefer, of film are going to get fogged or wound past the film gate in actually threading the back? Are any frames wasted on the end? What's a safe leader and footer length as far as the lab is concerned? I don't want to file perf rollers off the top fo the film because I would like to have the images aligned evenly on the film, but is there anyway to reversably convert the back to taking unperfed film without any consequences like scratches, kinking, etc.? How much film can I jam into the RB back safely with a cassette? Is it possible to fit more in without a cassette? I'd like to get 15 feet in a cassette in the back, but I can't afford scratches or jams in the line of work I do. How many sprocket holes wide is a frame of 6x7? Is it 15 or something smaller? Thanks.