Rotating a circle to a specific point

What I’m trying to do is make a dial-type of interface, where you click on a point on the edge of a circle and that point will rotate to another point at the top of the circle.

What I can’t figure out is how the make “Point B” rotate to “Point A”, where “Point A” is always fixed at the top of the circle and “Point B” (and Point C and Point D, etc…) are located along different points on the edge of the circle.

I’m using the tween function to rotate, so what I need to figure out is how to get the difference in the rotation between the two points.

I should have paid more attention in geometry in high school.