How to align pivot to the center of a hole

I've got a button model, that is supposed to be rotate around the pivot point in the center,

But how to set the pivot point to the exact center is a problem, the center should be further into the hollow area

I know this model is simple and if you create it from a cylinder the pivot is what I want, but I want to know if say I got model from other people and the pivot is not in the center, how can I align it to the center,thanks.

Adding to John Eason's answer: don't forget to set your pivot point accordingly Numpad . > 3D cursor
You could also pivot around the object's origin: Numpad . > Individual origin But then you also have to set the origin correctly: After setting the 3D cursor in Edit mode, you can set the object's origin to the 3D cursor when you're back in Object mode (right click > Set origin > Origin to 3D cursor).
Also helpful in this regard are constraints, e.g. a Limit Rotation constraint so you can only rotate the knob around it's z-axis. Also limiting the amount of rotation is possible with that constraint.