James, you didn't say what frame size your DH is, but I think your numbers are just about right for a 2 Frame gun.
Coming up with a reasonably valid number by halving the difference of the ID and OD, will only be reliable at the chamber, forcing cone and perhaps a few inches. Generally, you're more likely to find that, due to striking further down the barrels, the concentricity is less assured. I've seen differences of several thous between one side and another, or top and bottom, at the last 10" or so, then becoming more concentric around the beginning of the chokes.
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