Which Rule of Thumb is most “Important”

Of the general “rule of thumbs”, is one more significant than the others? Or are they all equal? The rules I am talking about are:

30 - 1 year salary saved
40 - 3x salary saved
50 - 6x salary saved
67 - 10-12x saved

My question more so is, do the later goals get easier if you hit the earlier ones, or are they all equally as challenging?