Good Post by PJ2
PJ2, and excellent post from someone who obviously knows their stuff. We used the x100 rule in the Mirage III O to work out other fighter's relative altitudes, and I often use the "short of the ground-line" technique to spot buildups ahead. Put the near edge of the groundline at 80-ish miles and wait for the nasties to appear just inside that. Over the water, it's easier.