I measure the center -in three places - average that length - I cut my borders a couple of inches longer than that length - BUT I measure the exact length on the border pieces - AND MARK IT - I also divide the border into (at least) eighths - and mark them - and I also divide the top into (at least) eighths - and mark them - and match the segments. So if some easing is necessary, it doesn't end up in a glob at the end.

Repeat for the other two ends - except this time I include the borders in the measurements.

Measuring has worked out much better for me than "just sewing on the border" - especially if there are a lot of pieces next to the border - they tend to stretch or do strange things for me.

I recently had this problem and received advise from two friends who are accomplished quilters...remove the border(s), measure and pin as mentioned above....if necessary steam the heck out of it (the border) and possibly any offending bias edges. Spray sizing also helps. This process worked wonders for my quilt.