If you wanted to be an absolute purist or if the offset were considerable, you could subtract the amount of offset from the centerline from the flange to center distance.
But if you consider that the change in spoke length is only about 1mm for every 10mm of flange to center distance another millimeter or two more or less won't be enough to worry about anyway.
The only time I'd make the adjustment would be if I was using an offset rim where the spoke holes are moved left to reduce dish.
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