There are a variety of ways to deal with stretch/flare. I'm often sewing a bit tight or scant and so my overall measurements aren't always correct as designed and in those case I really do prefer to fit the border to the existing condition. Other times, I want a strict mathematical answer and sometimes must either ease or stretch a bit of either the border or the quilt. For convenience and having short arms, I typically measure the seams or other points for a straight line about one-two blocks in, not at the edge.
If you google on "how to measure a quilt for borders" I had a number of things come up. This one is similar to how I do it:
https://www.youtube.com/watch?v=JhRzGr93WIo
There are also considerations on squaring your top. While we try for true right angles, sometimes projects come out more slightly kite shaped. I find for good borders you need good corners, and that is what a big square or triangle ruler does well.