Mine saved two more than required.... there are certainly several ways of passing this one.
EDIT: Lost 2, and one of them didn't really have to be lost, I was just lazy.
EDIT2: I confirmed that 100% is possible, but I give up on doing it. There's a nasty bit of precision that I just can't seem to get right.... (unless at least one lemming has already died >_<)
EDIT3: Actually I'm still trying.
EDIT4: DONE!
With one climber, one digger, 3 seconds left.