The best answer that I have seen to specifically answer your quest as to the optimum choke constrictions for a .410 S x S is somewhere on the web---but I'll be darned if I can find it now.
At any rate, the authors of this exhaustive study recommended specific constrictions and loads at all 8 skeet stations. In conclusion, they recommended the ideal constrictions in a S x S should be .009 in the right barrel---that's right, the right barrel and .006 in the left barrel. And choose which barrel to shoot (providing you have a DT or SST gun) first by the presentation--whether it be outgoing or incoming. They assumed 1/2 oz. shot @ 1200 fps and recommended using #9 hard shot.
Here's an additional article, admittedly not as informative as the article I cited above.
http://www.clayshootingusa.com/html/archive/jun_jul11/410%20gauge%20Choices.pdf
