that formula is a guide, but i'm not a big fan of "per gallon" formulas.
example, you have a 75g, capable of taking 18 inches of fish. would you put a 15-inch angel there? pasok naman sa formula dba?
depende sa fish. it would be handy to have test kits, para ma-monitor mo kung overloaded ka na. also, read about swimming spaces and compatibility.