I like the Open Look as well.
As for lb per gallon rule that's a very general rule of thumb. I'd try to come up with a general "theme" for the tank and make THAT happen. I find the look of the rock VERY important because if you aren't happy with the look it doesn't matter if it's 100lbs or 5lbs. It's not going to stay.
Also right now the "ideas" of rock and quantity are up in the air. We have the tried and true school of (avg) 1 - 1.5 lbs per gallon. Some are now saying that's over kill and it needs to be cut back. WAY back. I'm shooting for less in my new tank. I'm thinking when it's all said and done I'll have maybe .5 - .7 lbs per gallon. I'm going for LOOK. Remember some of your biological filtration will take place in your sand bed but that does NOT take the place of Live Rock. Have you decided on how DEEP you want the sand bed to be. That will also affect the "Fullness" of your tank.
Keep in mind that you're working with a cube which means you may have VERY tight size restraints when doing your design. Maybe take a piece of paper, sketch out your ideas and see how they "look" when incorporated into the CUBE restrictions.
Good luck and above all else.. ENJOY the process....
