I have done a few quilts with piano key borders. I love the look!
Usually I am not working with a pattern. I like to work with even measurements, so what I do is to measure the center part of my quilt. If the number is odd or fractions, I work out on paper what measurement would give me a nice round number say 3-5" more than the center (finished size) and I adjust the size of my inner border to get me to that point. You can always make it a little bigger, then trim it down if you need to.
Mostly I have 2-1/2" strips that I am working with and I will either strip piece them, then cut into segments the width of my border (plus seam allowances), or if they are a bunch of scraps, I'll just assemble a bunch, then trim them down to the width I want after. I like to do segments that are 10" finished, so 5 strips. It's easy to calculate how many I have vs. how many I need. When I get the majority of a piece done for a side, if I have to add a few strips to make it the right size it's easy. For starters, it might be easier for you to use a corner stone rather than miter the corner. If you are off a tad, use a slightly wider/narrower strip and it won't be noticeable. Another option is to use random size strips. Then you can adjust much easier when you get to the end. I recommend stay stitching around the piano key border when you are done so it doesn't start to unravel.
If you have a particular quilt in mind that you want to add a piano key border to and need help with the math, give a shout.
