I struggle with the math part of piecing sometimes too. No worries!
Top & bottom measure 57 1/2 wide? Is that correct?
Are you putting the seam in the center of the Top & Bottom border?
I would simply do the seam piecing first. Then measure out from center seam line 28 3/4 inches each way. That will give you a final width of 57 1/2 inches. It does look like your math is correct. Hopefully, others will confirm it too.
Pin each each end and the center together with quilt. Then add more pins. I typically pin the borders about every 5-6 inches which help prevent those wavy borders. Another tip is to stay stitch right inside of the 1/4 inch seam line of each piece and then press. It's another method that helps prevent wavy borders. It really only adds about 5-10 minutes to the work of putting the quilt together but gives me better results.
Good luck and have fun!