I have decided that to make a 13" hoop I need to make a bending form that is 11" across. My 10" form resulted in a hoop that was 12" across, so it looks like I need to subtract 2" from the final size when designing the form.
I won't bore you with the details this time. I did the same stuff as I did on the previous two forms with circles and screws and angle irons. Here is all the stuff.
The Stuff
Drawing circles on plywood, etc.
Drawing Circles Again
You know the rest. I ended up with a form that was 11" across.