No good pics sorry.
The simplest way I know to describe the cut is:
With either side up--trace the main rib bone to it's end with your fingers.
You will feel a soft area that is the start of the grissle area.
Slice down that "soft line" you found.
We have had spares that trimmed out to only 4.5 inches or so--could not tell them from baby backs at first glance without looking at the curve of the bone.
Maybe someone has better advice.
"Flirtin' with Disaster" BBQ Team (RETIRED)
FBA and KCBS Cook and Judge.
Former owner of a WSM, a Smokey Joe, a Charbroil Commercial gasser (junk), the legendary "StudeDera", a Fast Eddy PG500, and Sherman.
Now cooking with a Yoder YS640
Proud Pellet guy cooking on real wood.