I did that years ago and every brand I had lined up with the rest, an inch IS an inch.
Where our 'why is the block off" problems come in is the line itself on the ruler.
If you normally cut right on the line with a yellow olpfa (?sp) and then cut some more with a ruler with thinner lines the seam allowance can get off a thread or two also.
I figured out with the Easy Angle rulers Bonnie Hunter recommends, I need to cut those with one thread visible on the left.
