currently I have a tank with eco, just amazonia 2 and then amazonia 2 and power sand. I'd definately recommend that you buy special power sand. It will from day 1 keep your water clear. I had a fog with just the amazonia 2 for more then a week. Also anytime I disturb the substrate I get a little of a fog. Doesn't happen in my tank with special powersand.
you can always calculate the volume of substrate you wish to put in your tank in cubic inches just by taking some measurements with a tape or something...then take that volume and google "unit conversion" or something like that and it will pump your inches into liters. very easy. also, i would recommend amazonia I instead of II.
I have amazonia 2 and a lot of the black has gone away exposing the orange. It has lowered my ph to 6.8, which is what I wanted, but I also wanted a solid black substrate. My ecocomplete has worked out well for me- great growth, solid color, and does not fog the water up for a day every time it is disturbed. If you are raising CRS then go for the A2 otherwise get the less expensive eco.