Dan Harrington discusses this in his 2nd book (The Endgame), and calls it "Effective M". In an SNG or at a FT, you could calculate your Effective M by taking your normal M, multiplying by the number of players at the tabling, and dividing by 10-- he acknowledges that most tables have 9 original people and not 10, but that makes the math more difficult where precision is not as important.
For example, if you are on the bubble of an SNG with a stack of 2000 and blinds at 100/200, your M is 2000/300 = 6.67, but your Effective M is 6.67 x 0.40 = apx 2.67. This may seem very small, but considering how quick the orbits now are, you really have very few opportunities to get your stack in before your all-in becomes a trivial call for your opponents. Also consider that in this situation, you're probably the short stack, which should also impact your decision making.