Originally Posted by Mamagus
I suggest you sew the binding to the BACK and bring it to the front, then stitch it down on the front. That way if the back is a bit off it won't be so noticeable.
If you use a fancy stitch, it looks like part of the pattern.
As for the math: If you start with a 2.5" strip, then you'll have a 1.25" strip after pressing it in half. if you sew it down on the back with a 3/8" seam allowance, that should give you approximately a 1/2" binding on the front. (For me the theoretical math and the practical application never work as planned.)
I would fold the binding in 3rds and measure where you need to sew it down.