I don't know why I would need a 1/4" walking foot for binding anyway. I always sew 1/3 of the way in from the outside edges. So, 1/3 of the binding is in the seam, one 1/3 on the top and 1/3 on the bottom. I usually cut my binding strips to 2 1/2" but if making a small tablerunner, I sometimes cut 2 1/4" for a smaller binding.

It's probably going to vary a little bit depending on which foot you're using...I use the 1/4" foot with the blade that came with my machine (also a 6600) and for THAT foot, I set my needle at 4.2 to get a good 1/4". But I'm not positive that will be the same for you. You'll have to test & check but that will probably be a good starting point. The key is to WRITE IT DOWN once you figure it out! I use post-its on the inside of the box that holds my feet to mark down needle positions. I actually reset my machine so that it automatically goes to the 4.2 setting when I turn the machine on, I have to punch a button to get it back to center, but since MOSTLY I do piecing on that machine it's perfect for me.

On my Janome 6600P, I put the needle position at 5.6 when using the 1/4" accufeed foot - it gives me a scant 1/4" which I like.