Select the face and snap the cursor to it (Shift+S-->Cursor to Selected). Then delete the face (press X).
Press Shift+Num 7 to align the view to selected face. In Edit Mode add a plane and align it to view (check the Align to View box in Tool Shelf).
Scale the plane to achieve a desired size of the hole and connect the vertices of your helmet mesh to it. Next delete the newly created face.
Now you've got a regular hole.