I think the 1lb per gallon rule is good for most tank designs. The question is really one of footprint.
For example, a 55G has 73% of the volume of a 75G tank, but only 54% of the footprint.
So you need almost twice as much sand for a 75G than you would a 55G. But if you had a 90G, since the footprint is identical to a 75G, than you'd be fine with the exact same amount of substrate.
I have 80lbs of sand in my 75G, i wanted a relatively thick area so it could cover my egg crate that I put in to better distribute the weight of my rocks. Also, my Acei really like digging and I figured that giving them something to play with doesn't hurt.