If Pennington goes on IR, about 0% because then we have an easy roster spot for a new QB.
If Pennington's second opinion is that he'll only miss 7 or 8 weeks, then about 10% because you just can't say that Henne will be 100% healthy.
There's about no chance that White starts over Henne on merit, though. We've let him attempt 1 pass all season, we clearly don't have faith in White to run a complete offense.