This is one of my favourite patterns and never given me any problems. Have you remeasured everything at lots of different spots to see if that will pinpoint the trouble area. Maybe post a photo and see if that gives us an idea. Being off by 1/2 is a lot - there's got to be a reason we aren't seeing. You've already cut your squares down - I've always worked with the 6" squares and things worked fine - this is a puzzle. I know you said your seams appear to be 1/4" but have you actually measured them and the same with the strips. You have that fancy ruler thingy to cut the strips but did you actually measure them with another ruler to check for accuracy? A lady at our guild has that same ruler/cutter slot thing and she did make a goof when doing a demo. Just thinking out loud here, you know.