Can you figure weight of solution with specific gravities?
Yes. Multiply the weight of a gallon of water by the specific gravity of your want to be alum solution to get the weight of a gallon of 48% w/w alum sulfate. Multiply that by. 48 and multiply that answer by 8000.
That's how much anhydrous aluminum sulfate you will need to use. If you are purchasing the octadecahydrate, multiply that number by the ratio (666/342). If purchasing the hexadecahydrate multiply it by the ratio (648/342).