I agree with you that 67-70 days is normal. The sonorans below hatched out on day 70. Once the first egg is pipped I will slit the rest the next day or two. Most of the time it is only 1 or 2 that are left to pip. Just take caution that you don't get too deep, obviously. If you go to the seowing isle of any dept store, they have tiny scissors that are very sharp and have very small blades. I like these just because they are easy to handle and maneuver and take the guess work out of how deep you may be with a blade. I do start with a little prick of a razor blade but just enough to get the tip of the scissor started and then just snip a 1" slit in the top. My two cents and hope it helps. Take care!
Jeremy
You did not state the condition of your eggs. Have they begun to crease and dimple? My experience with pits(northerns and jani) is that 67-70 days will pass until they pip. I would not get too worried on day 60. Now once they start slitting their eggs, the question should be.....how long do I wait on the laggards before slitting. I have had eggs with full term babies in them that never pipped. I've always wondered if I should intervene once two days have passed since the first of the clutch has pipped. I sure would like to hear how other breeders handle this scenario. I had a couple of potentially screamer jani not make it out of the egg alive last year.