In other posts I've mentioned that I've taken training from the good folks at Adaptive Defense Concepts, LLC. They-like Rex, have regular jobs but provide training at a nominal cost.
One of the guys on another FB page posted a range estimation puzzle, which got me to thinking about the class I took from ADC. I don't believe in coincidence, but late last eve, on ADC's FB page, the second of two articles on range estimation appears.
For those of you who like knowing the why with details, read both. If you want "just the facts, ma'am", read the second.
Or, you can just read here that beyond 600 yards or so, range estimation gets harder because of the limits of the human eye and scope limits. ;-)