I also have a Pfaff Expression 2.0. My 1/4 inch seams used to always be just too big, even when I moved the needle in a notch. (It looked like I needed to move it in two notches, which would have the needle hitting the side of the foot.) I bought a new 1/4 inch foot and that fixed the problem. Maybe that would work for you?
It turned out that the one with the flange was wrong. The flange was just attached incorrectly. It left me with a seam that was too big. I am now using one without a flange, and it is working well for me. Probably it was just that one individual foot, and any other 1/4 foot with a flange would do just fine. (How do I know the flange was attached wrong...my husband told me after he compared the old and the new foot. He even said he could fix it, but I have a foot that works for me now. No need to mess with the other foot.)
Hope this helps....and I hope you solve your problem. It is sad to not get the seam right when you are trying so hard, and if buying a new foot might fix it, wouldn't that be nice.
Dina