Rotation Around Custom Axis?

Yesterday, I managed to make movieclips rotate around a custom axis (not around the registration point) like this:
Basically, the registration point is in the center and I made it so that it’s _x and _y properties would change in such a way that the top would be attached to the desired point.

Well, I’ve come across a problem of a slightly different nature and I just can’t figure out how to do solve it!
In this case, the registration point is on the top-left corner and I was wondering how I would be able to make it appear like if it’s rotating from the middle using the same base principles as with my example above (using sin, cos, tan, etc…)

Here it is:

I have attached the source file underneath.

PS. I’m sure I could find other ways to alter the registration point in this case, but I need to know how to do it. It’s quite frustrating.