IOS: How to draw and calculate position of the custom callout arrow in IOS 6?

IOS: How to draw and calculate position of the custom callout arrow in IOS 6?

I am trying to implement an application which has a map and on that map
there are several pins. Also I am implementing a custom callout view for
the map annotations by sub classing the MKAnnotationView class.
Every thing is working fine but I am unable to draw the callout arrow and
cannot calculate the exact position of the callout arrow so that it shows
right on top of the annottaion pin. Any help would be greatly appreciated.
Currently I am just adding my custom callout view as a subview of
MKAnnotationView and altering its frame. But cannot draw and position the
callout arrow on the map.