Ohhh, I see, you're looking for someone to check your math. For some reason I thought you got the 48lb/foot(cub) from someone directly, but as I look back in the thread I see this isn't right, sounds a little light anyway.
If we are going to base it off on bheron's specs, we would have to first determine how many square feet of sand he has in there. First, we determine how many square inches he has... that is
72" X 24" X 1.5" give or take (just took the avg between 1-2, this is the most inaccurate dimension, since I think he's just guestimating)
= 2592 cubic inches
How many cubic inches are in a cubic foot? This is 12 cubed (12 X 12 X 12) = 1728 cub inches/cub foot
2592 / 1728 = 1.5 cub feet in bheron's tank is taken up by sand
How much sand (in weight) did he use?
= 100 lbs
100 lb / 1.5 cub ft = 66.66.. lb/cub ft
That is based on bheron's measurements, but (no offense to bheron

) personally I would just go with RC's Caribsea data of 70-90lbs/cub ft, however that was obtained, it was probably more direct. If bheron was off a little bit (as many of us are), the back was a little sloped or whatever, it would make a big difference.
PS: If the arithmetic ever gets confusing (and it does for all of us in this day of automation), just write everything down in ratios, and write out your labels for everything in the numerator & denominator. That way, you just match numerator with denominator and can cancel across.