I'm trying to model a pen holder that came with my graphic tablet. After editing and extruding a UV sphere, I end up with a flat bottom that's not perfectly circular (because of some proportional editing I did before extruding). I've highlighted where the errors are in red. How can I make all the vertices on this flat bottom equidistant from a given center (and therefore aligned in a circle)? Here's the Dropbox link to the file.
I've tried to scale all the vertices to 0 while in Bottom Ortho view, but I can't scale them back up to get the equidistant points I want. The only other alternative I can think of is to extrude a part of the mesh while it's still circular.