+1 to WolfsBane's post.
IMO the ideal setup for CX is identical stack and reach as your road bike, then flip the stem up.
A couple of quirks perpetuate the "size down your frame" myth. In order to have enough tire clearance in rear without lengthening the chainstays too much, most CX bikes have a slightly steeper seat tube, which shortens the top tube. Similar in front: longer fork means shorter headtube.