Like Metarinka said above, the basic formula is used to resolve the inside diameter. This would be simply pi*Dia=Circumference. When you have to weld the two ends together, however, you have a pair of problems to contend with. First, if your roll former is not true, the seam will not be uniform (this doesn't affect your particular situation because you're doing rings). Second, it goes without saying, the flat or "tongue" will never become exactly the same radius as the rest of the cylinder. This is a case where in theory, you can get a number down to the molecule, but in practice you'll just end up frustrated. With steel, you can get very close because it doesn't become work-hard like say aluminum. As a matter of fact, with your low carbon steels the more passes through the rolls, the better your form is going to be. I have found that, using the NA as a starting point, I can get just about anything under an inch thick to within 1/16". So Pi*(dia+T)+L (flat). Be sure to include the entire flat plus the beginning of the form, as this area will not be the same radius as the rest of the part (you'll have to eyeball it, there's really no way to figure that out mathematically). Also, try to work from the middle of the rolls. It may take a little longer, but it's good practice for keeping the rolls true. The top and drive rolls are slightly tapered to allow for equal forces across the face of the rolls, so the ends will close up faster than the middle. Those grooves on the right end, by the way, are not for rolling bar; they are for rolled seams in sheet metal duct work. :)
I'm glad you're giving the effort to teach our young ones, and I hope you keep it up.