I created the new stem piece from the existing faces but then, to cut down the number of faces, used the new stem’s dimensions and location to make a new cylinder this time with only 12 sides and put a ngon of the bottom. I may have overdone things a bit as you can see some edges in the rendered view.
Great way to keep dimensions accurate rather than guessing. Duplicate and separate! I also liked the lean idea of checking how the geometry looks when it’s rendered as you build rather than wait till the end. Something new every lesson it seems. 