Watts per gallon rule

Everyone says the reference point to get lights are base on watts/gallon.
My question is how tall does the tank has to be in order to use that rule.

Does 3 W/G become 6W/G when you place the coral in the middle. For example
if you tank is 18" tall and you place the coral at 9" will you double the
light on them? how about at 15" up?

I ask because I've been seeing those nano cubes with 18-30 watts with nice
coral set up.

TIA

Ruben