Depends on what your doing with the tank.... By that I mean is it going to be a reef? if so you could get away with about 21-22 LBS as the rest will come when you get corals which is usually attached to LR ... If you are going with FOWLR you will want the full 24 LBS.. Another thing to look at is the rock densitity as well
keep it open and let your corals create your aquascape. i recommend building a basic structure, but I certainly wouldnt fill it up with live rock. Let the waving tenticles and growth patterns of your SPS make it up for the most part. Pagodas make great ledges and natural overhangs, as do caps
to answer your question more directly, I would start with about 10-15 lbs. Mixed fuji for the base and Marshal Island to make some caves and such
OH BTW .. cubes are a little tricky. I have a 65 G cube display in my living room that I love. Its difficult to make a structure and still have adequate room aroun the base on at least 3 sides. I'll try to post a pic of my most recent aquascaping venture to give you an idea.