See PDF. Note that this is just an idealization. In real life every site is analyzed on a case-by-case basis to see where shadowing by things like hills, valleys, and large clusters of tall buildings and the locations of towers are decided from that uniquely generated map to get total coverage.
It's just a ratio or factor though so it doesn't really matter whether it's upside down or right side up. As long as you know what it means the way it is written and use it correctly in that context.