Determine Cell screen coordinates
 

I am programming an excel add-in using C#. I see a cell has TOP and LEFT
properties, but these properties return the coordinates relative to the top
of the "grid".

How do I translate these coordinates to screen coordinates? I want to
display a new form right next to the excel cell.

So far everyone suggests manual methods like adding the heights of all your
command bars etc... but these methods all break down at some point (i.e.
when splitting windows). Surely there must be an easier way?

Regards
Willem