You are right if you are only measuring the front of the tank, but I have filled many tanks with sand and know it takes 7.5 - 8 Lbs per sq.ft. per inch of depth, depending on the type of sand. Hence I stand by my estimate of a standard 75 gallon - 4'x1.5'=6sq.ft.x7.5=45Lbs.A pound per inch thumb rule doesn't work. You could have a 10 gal tank with less than 2 sq ft or a 2' x 6' tank with 12 sq ft.
It takes about 45 lbs for every inch of depth you want.