One way that I know to make an easy mitre corner with strips is to make the corner a block. It would be similar to the method that Bonnie Hunter uses to make her strip twist
block.
So if your piano key border is 6.5" unfin you would make both of your strip blocks 6.5" unfin, lay them on top of one another RST, sew your seam diagonal from one corner to the other, use your ruler to measure 1/4" seam allowance from the diagonal seam and cut, press you block open and place as the corner block for your mitred corner look. Easy peasy.
I hope this makes sense. Your mitre block would look similar to the below. Each corner will need different orientation of the mitre block.
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