I bought 2-40lb bags of STS and had enough to do a 75 gal, 29 gal, 20 long and still have some left over, maybe enough to do my 10 gal but not real sure about that. The 29 gal I got a bit upset when planting after filling 3/4 with water and ended up dumping a bunch more in. Probably too much as it's almost 4" deep in some spots which I'm hoping won't hurt anything. The 75 gal is probably 3" deep if it were flat. The 20 gal seems to only have about 2" depth.
So after all that 2 bags might be enough, depending on how deep you want the substrate.
I'm not sure what it compares to using the calculator. I can't remember for sure what I used (think it was flourite) .. it said I needed about 1-1/2 bags based on 2" depth, so I bought 2. I'd already mixed them together when Diana posted she used 1 bag in an 88 gal tank and said: "Yes, the 40 lb was barely enough. 5' x 16", and the substrate would be barely 2" if it was flat.". I'm not sure how much I actually used in my 75 which is 48"x18".
Also, for me 2" doesn't seem deep enough for some plants but seems to work good in my 20 long at that depth.
Hope this helps some.