oh I get it, you are asking how much dry you will need for your project. Its going to be hard for anyone to recommend how much dry you would need for it as none of us really know the dimensions and or what pieces you will find, you could buy your 20 lbs of live and buy 5 lbs of dry and start messing around, or just buy the 10 lbs of dry and if you have extra start another tank!!!! lol just joking.... but not really... lol
Steve