Hey PC...here's wat i would do. U have two good choices. U can use the 2.8cu ft. of HP and add 1cu ft of the bx or just use the 3.8 cu ft. of bX and add some Perlite to it. Thats the only difference between the two. If u do use the BX bag (proper amount) DON'T forget to mix in a bag of Perlite to supplement it FOR DRAINAGE! Cheers PC.