This is from your link:
One more note on frame size and geometry: if you cannot move the saddle back far enough to get your knee to the neutral position, you should look for a frame with a more relaxed seat tube angle, or consider a slightly larger frame;

I think that's where my problem lies. I try to get as far back on my saddle as I can, but it is hard to stay there and my seat is all the way back. I wonder if I could correct it by using a set back seatpost, rather than getting a whole new frame?