I reached out to my Tech Rep and he replied:
"The 3GR is built around a 100 yd zero for common centerfire rifle cartridges, with each line in the tree below it extending down 1 mil from the previous line. The 2nd, smaller illuminated aiming point is at the 2 mil mark, which corresponds to roughly 400 yds across a variety of common semi-auto rifle configurations and loadings.
While the secondary illuminated aiming point will be 12 mils below the primary, or roughly equal to sight height above bore inside of 7 yards, this is purely coincidental. In practical use, knowing sight height on a target that close and appropriately holding over will be much, much faster than looking for the second, smaller dot. Beyond that, the primary dot is 1 MOA (6MOA on 1x) and the secondary dot is 0.8 MOA (4.8 MOA on 1x), and the larger primary dot draws your eye to it. In shooting closer targets, you won’t really perceive the small dot because you’ll be focused on the center of your field of view, and the primary dot. The smaller secondary dot gives a finer aiming point for greater precision at longer distances when you’re looking in the subtended part of the reticle.
The SI1’s subtends were done to be regularly spaced, with width increasing so that users aren’t “lost” in the reticle. It’s very easy to pick out the tops of the chevrons, and the increasing width makes it easy to tell which one you’re on. Kahles (and Swarovski) have tried to avoid bullet specific BDC reticles because unless you’re a mass consumer (like the US Government) and can guarantee that all of the optics will be going on identical rifles with identical mounts shooting identical ammunition it’s difficult to provide a BDC that is accurate and meaningful for the end-user.
Depending on the zero distance used, the particular bullet you’re shooting, the length of your barrel, etc there will be variances, but 5, 10, and 15 MOA will correspond to somewhere between 350-400 yds, around 500 yds, and around 600 yds for many common centerfire rifle cartridges. I’d recommend chronographing your rifle and figuring out both exactly where your trajectory will cross each of those holds, but how far in front of and behind that point that particular hold will be good for. As an example, an M193 55 gr ball loading out of a 16” AR (3050 FPS, roughly 2.7” sight height above bore) with a 100 yd zero will be on at 360 yds, 500 yds, and 605 yds. Another example would be Federal GMM 168 gr SMKs out of an 18” AR (~2515 FPS, roughly 2.7” sight height above bore), which will be 310 yds, 445 yds, and 560 yds with a 100 yd zero but 360, 490, and 600 yds with a 200 yd zero.
Running your ballistics will help bring a clearer focus to where those 5, 10, and 15 MOA subtends cross your particular system’s trajectory for a given load (or where the 1 mil subtends in the 3GR will do the same)."
I hope this helps
