At best for her, I can see 53% of the remaining delegates, and that's assuming 10-12 point margins in the big states without Idaho and Kansas-style blowouts in some of Obama's strong states.
That might though still be enough for her. Looking ahead I think OH/TX/PA push the balance of states from somewhat unfavorable to her to somewhat favorable.
More than just being able to hang on, Obama needs to have a more bullet-proof lead - like "even if MI & FL delegates are counted" and/or "even if Clinton's advantage in superdelegates is counted". That could be the sort of thing to compel the fence-sitting superdelegates his way.