I've been applying subtension reticles for downrange zeroing and rangefinding for many years. Now have a Burris 3-9X Timberline Ball. Plex reticle on my AR and have killed coyotes to 500-550 many times using subtension, and it's worked as well as anything else I've ever used for 1st shots. Easy enough to calibrate them several different ways some mentioned above, I usually calibrate them in 50-yd. intervals for elevation as well as windage, i.e. one reference may look something like this 4-2.2-.7 = 400 yds., 2.2 units below center x-hair, .7 of the plex post tip for 10 mph full value wind almost always at optic's highest power. Could also calibrate a magnification for a better trajectory to reticle fit, which is the fastest system of application, since magnification is inversely proportional to subtension, but that takes some study to understand--fun stuff though IMO. Have also shot many prairie dogs using this system on slow coyote hunting days, and it's worked great for me.