As well as -6dB for a doubling of distance, another great thing to remember is -20dB for a tenfold change in distance.
There are a few simple math things that audio guys need to know BY HEART and should be able to figure out to at least "close enough" .
If is not important to carry out to a couple of decimal places-I would argue that if you are within a dB or 2 of the "quick math" (as Art pointed out) it is not worth arguing about.
Just to add some more of the "quick math" things I use all the time.
100 watt=20dB 1000 watts=30dB.
So let's say that you have a 800 watt amp and you want to know how much gain it has.
You know that 1000 watts is 30dB and half of that is 500 watts (-3dB) is 27dB. And since the scale is not linear it is a bit "weighted" so 800 watts will be a dB or so less than 1000 watts but a good bit more than 500 watts.
The correct answer is 1.1dB less-but I would accept an answer of "around 29dB". Beyond that it is not worth arguing about.
If you can't figure out this in your head QUICKLY-you need to go back to school-sorry.
This is NOT that hard-with a little basic understanding.