If you were cutting a pinked edge with a rotary cutter, wouldn't you line the ruler up with the measurement you were trying to cut (say 2 1/2" mark) and cut with your pinking blade? Then the 1/4" mark would be measured from the "valley". Seems like it would be harder to sew along the valley though. I think you really have to measure your pre-cut to know where to sew your 1/4" seam.