I would agree with Noa and would like to add: a "rule of thumb" is a
"rule of thumb", doesn't mean it's logic but statistically viable.
It's based on best results acheived by most. The idea is to give an
easy relation between light and a common parameter, the volume. No
need for complex calculations and it gives good results!

If you wan't to find out more and understand i suggest you go on
thekrib, there is some info. It's related to a lot of factors but can
be calculated precisely.

Andre