But if they're going to try to come up with a rule of thumb for the general public - most of whom don't have a budget and have no idea what they're spending on various expenses now, much less what they'll be spending on stuff in retirement - then equating it to salary gives people something to consider.

This is one of those "least common denominator" type of recommendations, for sure.

Just out of curiosity - if you were going to give a suggested "how much you should have saved for retirement by age 40" guideline for the masses - who aren't going to spend more that about 90 seconds calculating an actual amount from the formula you give them - what would you use?