Mike ... I would suggest you use a fool proof method to insure rim offsets. I've done this on several GS builds and it works fine.
With the chassis up on a stand or building bench with the front and rear wheels clear, first adjust the spokes of the bare front wheel to center the rim in the center of the forks. Measure carefully. I've found it handy to make a wedge shaped piece of material, heavy cardboard, alu or in my case made from ridged sheet plastic. After eyeballing the rim, fit the wedge between the rim on either side between the forks for equal distance, at the same time insure rim run out.
Then use a pair of straight edges clamped to the front rim, using rubber faced cabinet makers clamps. With the straight edges measured parallel extending beneath the chassis that will fit alongside the rear wheel rim. I use two lengths of 1" square extruded alu tubing available from a local builders supply. Then adjust the rear wheel spokes to center the rim between the tubes and adjust for accurate rim run out. To compensate for the difference in the rim widths I've taped thin pieces of wood trim to the area of the tubes adjacent to the front rims.
This method may sound fiddly and time consuming, but a few 'cool ones' consumed during the process works for me.