craftgrrl | quilting math is frying my brain, lol

i'm trying to work out my first quilt. silly me, i bought the fabric first and now i have to work out the dimensions. it has taken me almost 30 minutes, but this is what i have worked out so far . . . will someone please check my figures and warn me if i goofed??

for the back, i ordered 2 fabrics, 1 yard each. i want a simple checkerboard pattern. (for the front, i've got 4 yards so i'm not worried about that, yet, lol).

if i make the quilt 4'x4' (48"x48"), my checker squares should be 8" each, 6 per row? that's 36 squares total, or 18 of each fabric, and i figure, with a 1/4" seam allowance, i can get 20 8" squares from each yard (or 20 8.5" squares). is that right???

i was hoping to make it bigger, but i think that's impossible on only 2 yards of fabric. if someone can work it out, though, please let me know. i'm going crazy >_< lol.