Although I have never used that brand, I will say that I have approximately 40# of PFS in one of my 40B and it gave me about an inch of depth. In my other 40B I have 50# of BDBM and it's maybe 1.5" deep. If all things are +/- about equal, I'd guess you'd want twice your initial estimate for around 2".
40 might be enough but I’d probably buy that third bag. I have supernatural in my 40 breeder and I actually used 60lbs on just the left 2/3 of the tank. Of course it was mounded up to about 4 inches in several places but I was surprised how much it took.