CAB,
I left out one of the important requirement of dimensioning isometric rolling offsets-the travel length.
It has been so long since I used a rolling offset that I was misled, and therefore misled you, with the images I posted. The need for the fitting angle is really important. Please look at the latest image and the attached pdf to see what my feeble mind is incapable of explaining.
For those that may not know, pipe isometrics are used for both fabrication and installation and are not drawn to scale. There is two conditions that dictate the dimensions of the offset. The first is the use of 45 degree elbows (butt weld, socket weld, threaded, soldered, etc.) and the second is to bend the pipe or cut welded fitting to suit.