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.