Don't cut the 2 1/2" inner border yet.
What I would do is figure out the length of the piano key border first. It might be easier to actually sew it first.
If you cut your charm squares in half, you get 2.5" pieces. Subtract 1/2" for the seams and your border should wind up being a multiple of 2" Plus 1/2" for the end seams.
Now measure your quilt. Instead of the 2 1/2" border, cut this piece the size you need to make it the size of your piano key border, plus 1/2 inch for the seams. This inner border is actually called a coping strip.
If you like pieced borders, you should get the "Border Workbook" by Janet Kime. She has dozens of border patterns and instructions on how to make the borders fit your quilt.