I used a depth gauge when doing the surgery. These were developed by David Gelb of Connecticut and I use them whenever I am working in the posterior mandible. In the x-ray on the top, you can see the penetration is just about 7.0 mm and I have about a millimeter and a half before I hit the canal.

In the x-ray on the bottom, I am using the 3mm side of a paralleling pin to check my distance from the canal. Still about a millimeter to go...