To range things at extream ranges requires a more powerful range finder. Newcon optics of canada makes every flavor of lazer rangefinder you can think of 3000 meter 4000 meter all the way out to 21,000 meter, but the 21,000 range finder is extreamly expencive $29,000.00 the smaller versions are a couple thousand, and in my opionyon worth the money. Another way is to subtend a known demention, the for example the avrage man is 20 inches from sholder to sholder if the redical in your scope is setup in minutes of angle you look and see how many minuts that 20 inchet takes up if it only takes up 2 minuts; 20devided by 2 = 10 x 100=1000 yards if your redical is in mildots 1 mil = 3.5 minutes or 1 meter at 1000 meters. The lazer range finder is quicker and easyer to use but if your scope has a moa or mildot you should do it both ways to double check and sharpe your skill at dementioning the range.
The biggest problem at long range is the wind, you should also get a good wind meater and use it all the time ,even when not shooting to become familyar with the wind and know its value, and how many minuts its worth at a given distance, pratice alot. other wise knowing the range is usless.