On backs I like to us 1/2" seam allowances - they press flatter and easy to spray baste flat.
This would make 11 1/2" finished blocks (if you cut 12 1/2" squares) unless you are starting with a 12" unfinished in which case it would be 11" finished.
12" squares: So if you did 6 across for 66" and 7 down for 77" you would need 6 times 7 for 42 squares.
12 1/2" squares: So if you did 6 across for 69" and 7 down for 80 1/2" you would need 6 times 7 for 42 squares.
Either way would give you plenty extra around the top.