Since you're talking about M values, you've obviously read HOH2. Another point that Harrington stresses here is your "first-in-vigorish". Meaning that when you're short stacked, it is most critical that you are the first one to enter the pot, as this allows you fold equity as well as offering you a wider range of hands to push with. This is an easy fold IMO since your hand isn't that strong and someone has already entered the pot all-in. This also incorporates the Gap concept, that you need an exponentially stronger hand to call an all-in with than to push with yourself if your'e the first one in.