I wouldn't worry too much about the pounds per gallon, just search for some really nice pieces to make a nice aquascape. It seems to many people just pile in a load of rock and end up with a rock wall in their tanks. Lots of openings and places to put coral is ideal
My two cents worth when it comes to aquascaping anyways
.. I should add that I think the lbs/gallon calculation is based more on the biological filtering of the tank rather than taking the visual aspects into account, I should ask if you are planning on running a sump and a skimmer? This would help reduce the amount of rock you "need" for bilogical filtration. You could also fill you sump with rock, thats what I have done in my tank. I think I have about 100lbs in my display tank but atleast another 50-60lbs in my sump.