that's when I'll worry about how much penetration would be needed. What you would have to do is develope a correlation between flesh and whatever test medium you are using. Darn hard to do because critters are not solid or consistant in their make up. If we all could hit them squarely in the ribs and always hit a rib bone on the way in, then we would have real data to draw upon. Then only for the bullet used at the velocity it was at and at whatever yardage.
I feel we make this more complicated that we maybe should. A well constructed jacketed bullet in the right place at enough velocity will penetrate enough to kill quickly. A hard cast lead bullet with a big flat meplate, will cause a lot of hydraulic shock. It doesn't need to expand. It will also penetrate deeply.
When I tested the new Hornady interbond, I used water filled gallon milk jugs lined up long ways. It's cheap, easily transported, not too heavy, and repeatable.
http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=14407&highlight=Again, critters are not solid water either. But you can get a reliable indication of expansion, weight retention and penetration.