Hi all, I have always had trouble doing a 3/8 inch binding so it's nice and even. I've been using the method in Alex's "finish it" book. This has you cut the binding 2-1/8 inches, cutting the quilt 1/8 inch outside the edge, laying the binding at the edge of the quilt so the 1/8 inch of extra batting shows, and sewing a 3/8 inch seam. I usually cut the binding 2-1/4 inches because I can't seem to get it exactly right. And usually my quilt isn't exactly even so when it's trimmed there isn't a good 1/8 of batting around it all. It's definitely my technique, not Alex's instructions!
Today, I sewed on a binding as per the instructions, eyeballing the 1/8 inch. After going all the way around the quilt, I noticed that the binding didn't fold all the way over to meet the seam on the back. I ripped it out, fortunately the quilt is only 35 inches square.
Then, I got a piece of stiff paper and drew some lines on it. There's an edge line, one line 1/8 inch away, then 1/4 inch away, then a final line that's the width of the folded binding. I used this for a gauge for the 1/8 inch, which turned out to be a lot more than I thought it was! I used the 3/8 inch line on my sewing machine throat plate for the seam allowance. This turned out to be the most consistent binding I've ever sewed, no wavering or dips. Here's a photo to make it more clear.
I've been quilting over 10 years and just now figured this out! Hope this helps someone else, Kathy