Bugs-
It's been quite awhile since I did that test, but it seems to me that it was about .75".
That may not seem like a lot, but I was testing about 10 knives at the time, including some fixed blades and that was about the maximum depth.
One of the knives I tested was a Cold Steel Scimitar, it was very disappointing. The lock was stressed so bad that it developed a huge amount of vertical play. I sent it back to them with an explanation of how it had occurred and they sent me a new knife. Of course, in the real world, even if you did hit a hip bone on your thrust, you've probably acheived your objective.
One of the big surprises was the tremendous lack of thrusting capability of most fixed blades. I found that most of the fixed blades I tried were lousy thrusters.
The dynamics of the thrust require a lot more than just a grip on the knife. If you are only considering soft target type thrusts you're probably ok, until you hit bone, which is what the telephone book simulated. Then, you find the standard hammer grip is just a guide to allow your hand to slide either up onto the blade, or if you have a guard, painfully into that guard. A saber grip with the thumb resting on the guard will result in a broken or jammed thumb.
Palming the butt of the knife gives you the surest support for the thrust. But most knife handles are not designed for this type of grip. The Military and the Scimitar handles are excellent for supporting the thrust though. The Military because the design of the handle allows for a very secure palming grip, and the Scimitar because of the pistol grip.