Ciao-
It's not a good idea to go by pounds per gallon rule with rock or sand. The weight of live rock varies greatly depending on where its from. If you can pick out the pieces that will look good in your tank. You will gain sufficient biological filtration from (what looks like) even a small amount of live rock. Think about the types of animals you are going to keep. Make the reef structure look the way you want and don't worry about formulas. If you are mail ordering pieces, ask the sales person about the sizes and types of rocks they have.
As far as sand goes, do some research on what type of sand bed you would like to have (ie. deep sand bed/thin layer/plenum etc.) Which type is best for you? That's where the research comes in. Just do a search.
Good luck!
