I'm impressed that I see no cactus in the pic!!
Oh its there, trust me.
I'ts the Law of the West: When antelope hunting in Wyoming, any time you set, kneel, or go prone, you're into cactus.
As to ranging, I do simular only I do use a Mil Dot. This was a 270 Win, 150 Grn Hornady IBs at 2800 fps.
I normally carry it zeroed at 275 yards, its never 4 inches high or 4 inches low to 425 yards. ( the vital area of the normal antelope is 8.5-9 inches). So you aim at the center of the heart/lung area, you should be good.
The size, top of back to bottom of stomach of the average antelope is 15 inches. If you fit that 15 inches into your scope, and a 1 Mil or larger will mean the animal is in range. (15 inchs with 1 mil is 417 yards).
On this critter, it was a bit farther. I figured 600 so set the scope to hit at 600 and the picture tells the story.