That is just a "rule of thumb", like the "Watts per gallon". Yeah, you really want as much rock as you can put in there for biological filtration purposes. However, if it is not pretty to look at, what's the point. Also, if you "cram it" and reduce overall flow around your reef, you'll have problems.
Personally, I put as much as I can, keeping aesthetics in mind, and remembering that most of the corals I put in there, will be on a small rock, and need room to grow. You also want to have room for schooling fish to scoot around in.
If you have a
sump, with a chamber that is somewhat empty, you can add more rock there for filtration boost. If not, do the best you can providing as much rock in the display.
Don't forget... the volume taken up by 1 lb of dense, Fiji Rock, is much less than a single pound of lighter, porous rock. Again, the "pound per gallon" is just a ball park range.
Dave
