I go by 1.0 -1.5 lb of sand per gallon for a regular sandbed. And the rock ratio is about 1.5-2.0 lbs per gallon.
These are just rough estiamtes depends on your rock and the depth of a sandbed you want in the tank.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.