How important is the rule that You cannot be off the answer for more than 20%?

It is not really important because it is impossible to get a correct number without using real data. It is about showing your ability to break down a problem, use numbers effectively and that you have a degree of showmanship. Because the thing about consultants is that they do not have some secret knowledge their clients do not have, but they need to convince their clients that they do. Suppose you just estimate the number from the start. In that case, you are most likely not worse off than if you estimate the different elements and do some fancy math before giving the answer but using such an approach hides the fact that you are just guessing behind some math and pretty graphics. And that is what the job of a consultant is about in the end. 

(edited)

Hi,

That's the first time I hear about that principle, so I can assure you it's not important.

What's more important is that the numbers should make sense. Eg if you are calculating the luggage carts at the airport terminal and you get let's say 10.000 - looks like the numbers don't make any sense and most probably you've done a mistake

Best