Originally Posted by Jennifer23

The best "no-guessing" way I know involves some marking.

First, on the wrong side, make where your 1/4" seam will be on the center strip (light blue in my diagram). Then, use a ruler to extend a line from the edges of the parallelograms across that seam allowance line (red in my diagram). The edge of the new parallelogram strip needs to go through the point where the two lines intersect when you put them right-sides-together to stitch.
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I hope that someone else suggests an easier way; I'm doing something similar, and would love an easier way to line things up!