How many bags Seachem Onyx will I need?

I have always read on here that the basic rule is 1 lb substrate per gallon of water. If you go by that, 15 bags would do a 231 gal tank.

What is the footprint of your tank? The width and lengt of the tank? I used 7 bags for my 48"X18" tank. It gives me ~2" up front sloped to ~4" in back.
There is a calculator for figuring substrate depth using "x" material if I can only remember...ah ha!
http://www.plantedtank.net/substratecalculator.html

They don't have Onyx as a choice, you can use Flourite to calculate and add a couple bags for the Onyx sand, or if you're using Onyx gravel stick with the Flourite calc.
HTH