This may not be helpful, but when I changed my middle ring down from a 42- to a 38-tooth, my tiagra front derailleur no longer shifted well to the granny. The bulge in the derailleur stopped too high on the outer cage to push the chain off the 38-tooth and onto the granny. On the advice of people on this forum, I changed to an old dura ace triple front derailleur that I got on ebay and that solved my problem. If that had not worked, I would have had to go to a smaller big ring in order to be able to drop the front derailleur low enough to work.