Of course, modeling clay, play-dough, water and some other substances are much better in opening up hollow points than real flesh (or the ballistic gel 10%).
I also am very interested in a consistent method to make those cuts.
I just use a blade for my utility knife. Use light pressure, rock it back and forth and I do it all on that green mat you see there. I just eye ball up center.
Jungle shooter, I use modeling clay. It's a nice consistent medium to shoot into. Play dough doesn't stop anything. Ballistic gel takes too long to recycle and too expensive. I can get 5 lb of modeling clay on Amazon for cheap
Here's a fun way to use modeling clay. One side is iron. But is you turn around the stand, on the opposite side they are filled with modeling clay. For a different effect. Thread 'My two sided targets' https://www.airgunnation.com/threads/my-two-sided-targets.1291060/