Cut a border strip 86 inches but mark the 85 and pin the 2 border ends at the 85 inch mark together. Now pin the center of the border to the center of the quilt end. Continue pinning at intervals along the edge. Place the quilt top on the bottom next to the feed dogs. The feed dogs will help ease the extra into the border strip. When you get to the middle, check the pin there. Are you down to a 1/2 inch out? That should tell you the second half inch should ease in.
I know most people would say cut the border and ease it all in but I like to give myself the extra fabric just in case you end up about 1/4 inch over. The border being out a little is better than wasting a whole border strip with it being cut too short. Do the other borders and if they all come out about 1/4 inch out, pat yourself on the back and trim off the extra border strip ends.