jc121,
It depends on the mirage.
The heavier the mirage, the lower I have to go on the zoom.
I can have it all the way up to 60x for the 200!
I keep it around 24 to 30 most of the time.
For the 1000yd line, I turn it all the way down, to reduce distortion.
On an overcast day, I can see .308 holes @ 600yds. .223 @ 300yds on a bright day with medium or lighter mirage.
Lew