You will have to allow for the selvedges and seam allowances on your backing fabric as well. If it's a true 45" wide you may lose 2 inches in the 4 selvedges, then another 1/2 to 1 inch in the seam. Your backing may end up being 87 or 88" wide.

When you say you want 1.5" borders, are you saying that is how wide the finished border will be, or how wide you will be cutting the stripes to make the borders?