I am not familar with the double numbers but it is according to how much trash you are willing to dig in the quest for gold rings. Idealy if you are hunting beaches is to dig everything from the lowest foil reading up to zinc. If you hunt in pea gravel do the same thing. Gold rings can show up anywhere on the screen or any number combo so it is hard to nail down exact meter readings but you can modify your search method from the above to something to make it easier, realizing that you may miss some rings.
If the area you search has the new style pulltab in abundance, simply ignore them when you get their signal. The foil range up thru nickel which is just under where the new pulltab hits, is a good area of the screen to dig, there are probably more thin gold rings lost than bigger ones and the thin ones are hard to see if it is lost. It is according to your search site, how much trash, what style pulltabs and other alum trash is present.
Another thing to consider is the specific smaller areas of where you are detecting, those smaller areas that may be more prone to have more rings lost. For instance if searching a playground, the area around swing sets, if beach hunting, the area around where volleyball nets were.