What about:
Form.ScreenToClient(Control.ClientToScreen(Control.BoundsRect.TopLeft));
However, Control.BoundsRect.TopLeft is the control's position within its Parent's client area. The coordinates provided to ClientToScreen() need to be within the client area of the control that it is being called on.
So, to use BoundsRect, you would need to call ClientToScreen() on the Parent instead:
Form.ScreenToClient(Control.Parent.ClientToScreen(Control.BoundsRect.TopLeft));