UCA bushing inside diameter question

Yes you are understanding it correctly.

The benifit is that you have created the possibility to run more caster and better balance the side to side numbers.
With the factory bushings, the alignment setting often have to be compromised because one side or the other has a limited adjustment range. Not anymore with the new bushings.
Most of us run camber in the range of minus .5* to about minus 1*. We believe this lets us corner a little harder without tire slip.We call this better handling. With typically sized and fitted street tires this is very difficult to prove. But we believe it, and do try to prove it by drifting every generous corner we find. Okay maybe it's just me.
To get the .5 to 1camber, we have to sacrifice caster. That's just the way it is. I have aligned A's to -.5camber and had only 1 degree positive caster left over.
Well caster provides high-speed stability. 1* is just not enough. However,3* to 4* is, at .5 to 1.0 neg camber.
But you still may have to sacrifice some caster to a lower number for a bad side, cuz the cambers HAVE to be the same. If they are out by much, they will contribute to wander.
So to summarize, the problem solvers will easily get you back to the factory settings, but will also allow some performance settings, like .5 to 1.0 neg.camber with 3 to4 or a tad more,pos. caster.
If you have manual steering, be wary of more than 4* caster, or parallel-parking will become dificult