It's arbitrary. I just put them where the flatter areas would be close. If I use the average values as the 0,0 points, the plot looks like below. The least change for horizontal is actually shots 6,7. and 8, while for vertical it is 8, 9, and 10. I would tend to favor vertical as it isn't affected by wind as much, but you can see how any of those are good candidates.
