Peggi - recheck your calculations.
It's very simple. Measure the length of the quilt (in several places and take the average). This is the length to cut the side borders. Sew them on.
Now measure the width of the quilt with those borders included. This is the length to cut the top and bottom border.
If you want to cut all the borders before you start sewing, you still measure to get the first measurement. That is the length to cut the side borders. Then add to this 2 times the finished width of the borders and that is the length to cut the top and bottom borders.