yea im with mosin on this one i dont quite understand your question.. your gun should have a choke tube in it check your local Gander Mountain or Bass Pro or whatever sporting goods store you have in your location but anyways go to the store and ask for a choke tub wrench it fits all size choke tubes. let me know if and when you find one.
I'm thinking that he doesn't have a choke tube but is asking what choke his 20 ga is based on the measurement he took. Is that a open, improved, mod, or full choke diameter? You math guys should be able to convert the measurement to choke size based upon known barrel data. I checked my 20 ga mod barrel at .610. Could be a loose mod or improved cyl choke. That's as close as I can get on this one.