I guess I'm not smart enough to know how to share a picture on here from my iPad but, in my experience, .575 to .580 is the perfect choke for TSS.
I have a .575 Carlsons in my youth 870 with 21" barrel and at 40 yards it was easier to count pellets OUTSIDE of the 20" circle so that's what I did...of 590 pellets in the 1 5/8 ounces of #9, only 41 were OUTSIDE of the 20" circle...so 550 inside it and they were very evenly dispersed, without any voids across that pattern.