I like David Marotta's advice on the subject:
http://www.savingadvice.com/blog/200...net-worth.html
Which would say, at 40, you should have 3-4 years of annual "expenses" saved up.
I like thinking in savings in terms of "annual expenses" instead of "annual income."
Likewise, if you are ahead or behind, it gives you a good guideline of a direction to move towards.
Of course I think this is rather vague as it focuses more on retirement savings than anything else. Refers to savings/investments.
(I think it is largely similar to the MMND formula, except it accounts for you would have way less early on and way more later on...)