How can I control the size (Diameter) of a ring that is made with the simple transform / Bend? I start with a 1 x 1 x 1 cube set at -9.5 Y units from the origin. I use an empty at the origin as my Center of Deformation. I use the simple deform modifier / Bend with 360 deg as the deform parameter.
The resulting radius of the ring is about 10.159 and the diameter of the ring is about 20.318.
Yes, I can scale the ring in X and Y back to the desired Diameter=20, and recenter the ring at the origin. But this introduces a slight error in the inside diameter of the ring that is a challenge to correct.

Is there any way to predict what this dimensional difference is before I begin and adjust for it in either the mesh center, cube size, Center of Deformation, or anything else that is predictable?

The Bend Deform is simple, but the outcome is imprecise. Is there a more precise way to construct the ring?
Different, slightly quicker approach -

Add a cylinder and change the radius and depth in the last operator panel to match the desired values:


Then in edit mode, select the top and bottom faces, extrude in place (press E, then immediately after Enter before the faces had a change to move anywhere), then change the pivot point to individual origins:


Now scale the top and bottom faces to match the desired radius. Since we change the pivot to individual origins, each face will scale around its own origin, and in effect both faces will be inset.


Now the only thing left is to bridge the two faces to create the inner hole and geometry (see answer above to explain how to activate the looptools addon if youre not familiar with it):