Subdivision Surface Strange face choice? I have been trying to model a pen clip back and running into a strange effect with the subdivision surface that I do not understand.  For some reason, the interpolated surface is dipping below the original geometry and then bending back up.  I can not figure out why it is doing this.  It is best shown with a picture.  



You can see the dip down on the arc to the right side.  I would have thought it would be a smooth transition from the arc to the first set of control points.  I attempted to add some supporting geometry with the highlighted line.  But that did not seem to help.  Any thoughts?  I am just not sure where to start. 

Blend File:



Updated Efforts

I have attempted to add some additional geometry.  This has fixed the original issue, but now I am getting some strange artifacts.  I will keep futzing with it but if anybody has a suggestion as to the correct type of supporting geometry I would love the suggestion.  Thanks again for everybody's help.  I learn something new every day.
The solution to this problem was to add the correct supporting geometry.  After some fiddling, I have found something that produced a nice smooth output.  What one should look for when this type of problem appears, as suggested by the comments on the question, is NGons.  Although blender has "full" support for NGons, the subdivision surface does not always play nicely.  Although I did not completely get rid of the NGons, I reduced the vertex count and the result is quite nice (I think).  Here is the result: