Help me understand SimpleDeform, Bend modifier?

I need to make a lot of circular or semi-circular objects (archways, circular walls etc) and the SimpleDeform Bend modifier is great for that. However, every time I use it, I have to mess with it quite a bit to get what I want, because it seems like the axis it bends on is off-center.

The first picture is using the empty as the bend point. It is directly in the center, which should result in a perfect bend.

The second picture shows the bend working correctly, but only after I move the empty to one side. The empty is now off-center, and I have to manually align to curved part by moving the empty, rather than it simply being lined up perfectly the first time.

Am I doing something wrong? This is a really powerful tool if I can get past this issue.

It would appear that point around which this modifier bends is not origin or target as one might expect, but something a bit more complex.
Found this:

…tbh, I’d just resort to Curve Modifier for this: has its own drawbacks but easier to control, imho