I have it adjusted where the outer ring has very few "Ports" burning on low and it is still too hot for a low simmer. On low there is a separate "Low End" adjustment you make to the valve (Like an idle mixture screw on a carb). I can adjust it so low that flames will go out. Bottom line is the burner cap has too many ports for very low temperature simmering. What I am trying to do is take one of the burners and make it into a low temperature "Simmer Burner".
One of the folks on a forum I frequent has blocked the two outer rings using Permatex Muffler and Tailpipe Putty. It works but looks like crap!