I zeroed the optic dead on at 100yards according to Trijicons website ( the top of the horseshoe ) and tested it at 200, 300 and 400 and it was adequate to make first round hits on the gongs and flashers I was shooting at. I threw it on my 20", checked the zero which was only slightly different and had the same experience. Soooo, in short, barrel length is a moot point for me. The quality, durability, optical clarity and reticle on that optic is outstanding, it is alotta money but I look at as an investment, absolutely no buyers remorse here. That being said, I did have serious buyers remorse after buying my Aimpoint comp M4, Eotech XPS2 and Leupold CQ/T, and I sold each and took a hit. I would have been $$$ ahead if I just sacked up and bought the ACOG first. Like they say... buy once, cry once.
Hope that helps!
gonna assume the above is a typo-if you zeroed a TA11H at 100 yds with the top of the horseshoe, there's no way you'll hit anything beyond about 200. zero it with the top of the dot at 100 meters. then, the dot holds perfect out to 250. after that, assuming your loads are matched to the BDC. the tree works for 300 meters and beyond.