Like others have said, depth of the cut and how much reed over hangs is most of the rasp. Also the way that you present the air across the call determines how much rasp is made. Myself, I use a 2.5 reed ghost, but I can make as much rasp and anyone out there, a lot of that is how I push the air across the call, how much I drop my jaw and some of it is made with the back of my throat. ( at least that I think it is)
Stretch is important also, but I don't think it is the most critical part.
I can take that same ghost cut and make 4,5 , 10 notes crystal clear, and when I want the rasp, the next note drops into that rasp.
Many callers are sloppy in how they push the air across the call. There is a huge difference in just having reeds flapping in the wind vs what natural rasp sounds like.
Cutting on a mouth call is the same way, some guys sound like a cheap rubber basketball bouncing on a hardwood court, they are sloppy with how they push the air across the call. Distinct and separated notes is what your looking for!!!