first of all, with metal thicker than 18 guage, to figure out the stretchout of the band u have to take the mean diameter because the metal stretches.  for example, 1/2 plate band with an i.d. dimension of 24 inches the stretchout would be i.d plus one thickness times pie. 24 + .5 x 3.14159 = 76 15/16".  To roll it, u back roll the first 6 inches (run it through the roll from the back roll through the pinch) to the required curve, (check it with a sheetmetal pattern). Once u get the first 6 inches, drop the back roll and bring up the front roll and run the whole piece through forwards till the end without taking it out of the roll. Tighten the roll up a little and run it through backwards, (backrolling is considerablly tighter, but this will give the other end the proper radius), all the while checking the piece with your pattern, until the ends meet.  I believe it is impossible to roll thick plate in one or two rolls without getting flat spots.  If you work at it a little at a time it will come out beautiful.