Most of the time I use a cone mandrel or the anvil horn to forge a ring but sometimes the ring must have a more exact radius. I found myself having to search the pipe drops to find the diameter piece I needed and decided to make two “radius towers” which were composed of the diameters I most often needed.
I laid them on an anvil and took and image which, I think, is self explanatory. I didn’t measure them today but I believe the larges piece of pipe is 4” o.d. and the smallest is probably under 1”. I can’t say how much searching time they have saved but it well exceeds the short time it took to make them.