I remember the sh!tstorm this caused in 4e
where they do it as a box for 3 dimensions.
So the error rate in XY+Z is nearly exponential.
I can understand rules wise wanting to have as simple a solution
to this as possible ; considering some of the people who play the game,
are not going to enjoy any geometry math class extra credit homework.
... I wanna rolEplay you !@#$ 's.
Hex is more accurate in 2D, but to really get it right in 3D
you have to do offset levels like the Packing Oranges
problem that high maths guys get to puzzle over.
I have no idea at all how to do this one ...
(wait you mean if I had paid attention to Math Geometry instead of Female Geometry in the exact same class ... )