I usually measure the "center" in three places - take an average of the three numbers (if there is a huge discrepancy between the numbers, there is a problem that needs to be addressed!)
I fold the top into halves, quarters, and then into eighths - and pin or put a chalk mark on the edge.
I usually cut the border "a bit" longer - hardly ever "exact length" - mark off the "length that matches the top" on the borders - I leave a bit "extra" on both ends - just in case - sometimes I need that extra little bit - and then divide the "length that matches the top" into segments.
Then I match up the segments. It's a lot easier to only have to deal with easing in a one to two foot stretch than worrying about the whole side.
This usually works for me.