With media like Orca Bio Cubes, Seachem Matrix, Pure Reef Stones, Bio pellets etc. you don't need that much liverock in your system. Thus leaving you more space for your fish to swim in and for your corals to grow out. That said, if you don't want to go that route then you need to go at least 50% of your tanks water volume for it to be effective. Live rock houses small critters etc. but its main purpose is to help with bio filtration (due to its porous nature) where it houses different types of beneficial bacteria. After time though as your tank matures a lot of the holes get over grown or blocked thus reducing the rocks filtering capacity. My opinion would be to shop around for a couple of nice pieces and then to go with Orca Cubes or Matrix stones in the sump to help with filtration.