Joe, I almost never sew with a stitch larger than 2.5 on my Viking, which I think is about 12 per inch, but I'd have to measure to be sure.
I did do the flannel on the bias since it's thicker than what I normally work with. Most often I use it cut WOF (because that has a bit of stretch). Mostly I think it's just that I've got a LOT of practice, like years and miles of stitching.