"What I'm getting at here is: I don't want to have to calculate in my head, when Drawing a certain-sized Object, where I have to start the thing in order to get its COE at 0,0,0."
Me neither, Alvin. To get around that, I create a small sphere and move it to (0,0,0). Then, when I want to create an object that'll have a Luminance, I snap to the Center of Extents of that small sphere to start the new object. In most cases, the COE of that new object won't be exactly at (0,0,0), but it'll be close enough if the new object is fairly small.
The alternative is to create the new object anyplace at all, then -- before doing anything else with it -- assign a new Luminance to the new thingie and plug the thingie's coordinates into the Location parameter of the new Luminance. This is probably easier than creating the new object at (0,0,0), but the drawback is that you have to create a brand-new Luminance for each new object.