It's kind of tricky. I just looked at mine and with a shouldered akront rim, the edge of the rim to center of fork leg is 2.25 inches. I'm not sure how to measure the actual offset. I'm not a wheel expert but have had some experience as follows. In the late 1990's I took my single sided hub to England while visiting on a job and drove to Hagon to have them build stainless rims and spokes. I carried them home and found the offset had not been included in the build. The wheel was severely lop-sided. I found "Spokey" formerly from Ghost cycles and sent him the wheel for truing (he was retired and living in New Hampshire). He said that he was able to get the wheel near the correct offset but the dimples had also been incorrectly punched (also without taking in the offset) and any more would have stressed the angles too much. SO it's important to not only know the offset before building, it's important to have the knowledge of how the rim holes need to be punched. I sent to hubs to Buchanan's and they came back perfect... so that's my recommendation... because it's tricky.
As a side note I did write Hagon once I was home and they were very apologetic, offering to redo the wheels while paying for all shipping etc. I passed on their offer want a closer and faster solution. additionally I also had bought new shrouded shocks then and after bottoming out one too many times realized they were 12.9 inches (late model OIF BSA) rather than the correct 13.4 inches (as they sell now). I just bought some ikon's (Semi shrouded)