There may be a fast way of doing this but I haven’t found it.
I often want to use an imaginary point half way between two others (to rotate, mirror, copy, etc). My workaround is to draw a line between the points, use the centrepoint, then delete the line. Admittedly, that doesn’t take long but it would be much faster to have the facility inbuilt or an extension that was dedicated to the task.
I appreciate this is mainly used in 2D drafting but if you consider any plane, horizontal or vertical, to be a 2D surface within a 3D drawing, the need is still frequently there.
If anyone has an elegant way to do this at present, better than my workaround, it would be great to hear about it. In AC, I had a keyboard shortcut that did this.