Measure quilt as you have mentioned from center top and center side. Add a 1-2 to 1 inch to measurement. Cut borders on straight of grain lengthwise.
If you are using multiple borders construct them as one unit before attaching to body of quilt.
Find the center of the quilt side and the center of the borders. Match. When sewing to quilt start in the center and sew out to end turn and sew to other end. You can then square the ends to the quilt.
If you are mitering be sure and stop 1/4 inch from end.
This method eliminates stretching borders to fit quilt which is what causes wavy borders.
Happy quilting Severna