Its pretty easy to lose a half an inch on a block where there are 8 seams. I bet the direction you are off on is top to bottom - there are 8 seams in this block in that direction. Make your seams a "scant" quarter. Don't push up against the mark you usually use for a quarter inch seam. Stay a thread or two away from it. Being off a sixteenth of an inch over 8 seams amounts to a half inch.