I may be way off on this I don't use wood pellets as the wood shavings are inexpensive but, I would assume you don't need the wood pellets to be as deep as wood shavings since they absorb moisture much faster/better than the shavings. A 40 pound bag of pellets should equate to 3.5 cubic feet compressed so I would buy 4 and see if 3 is deep enough if not add the fourth. The sweet PBZ would be used to supplement the pellets and so I would use a much smaller amount. I would use less than 1/4 of a bag. I use about 1/2 bag in each of my 10x10 horse stalls and it is very effective at that rate. Yes I know that is not a real comparison but, I use DE in the Coop right now (Sweet PDZ is hard to find in my area) However, you will really have to experiment. I would start with a small amount and add as necessary.