The M represents how many rounds your stack would survive at the current level if you didn't enter any pot, so it's only really useful in tournaments. It helps you determine when you should become more aggressive. If you're M is above 20, you can play tight-aggressive (if that's your usual style) and wait all you want, but as it gets lower your range for each position has to become looser.

In cash games the M doesn't apply. The blinds aren't rising and you can always add more

chips. How many blinds you have is what matters.

And M isn't difficult to calculate. Instead of using the normal formula of dividing your stack by the initial pot, multiply the initial pot by 5, 10 or 20 instead to see more or less where you're at. This way is not only easier to calculate but also more useful when you look at the other player's stack. If the blinds are 25/50, you know you need at least 750 to have an M above 10 and 1500 to be above 20. With simple multiplications you can easily see the situation of everyone at the table.