I took several photos and added a graphic.

1. Blurry - but measured to center-of-flange.
2. One showing measurement to inside-of-flange.
3. One shoring measurement to outside-of-flange.
4. Graphic summarizes the measurements in photos...
5. Added photo of Maillard 700s with extreme flange layout - just for eye candy.

Reason we measure to center-of-flange is so that we can provide a little extra length for the outside spokes without adding too much to the inside spoke length. 99.99 percent of the time - this works out just fine for traditionally laced wheels.

The center-of-flange measurement just so happens to be an average of the outside and inside spoke offsets...and the distance between the two typically is around 6.0mm for your typical alloy hubs with 3.2mm flange thickness.

We are describing 3 triangles here - each with different angles - which means they are not proportional triangles. One is a rough average of the other two.

We can use the ratio of 10:1 you mentioned earlier as a rough indicator of what happens to spoke length when you move a flange 1.0mm....but once you get past 40.00mm of offset...you'll see some creep to .11mm, .12mm, .13mm...for a given flange size.

Use the spreadsheet...play around...focus on American Classic (old), Bullseye, and Maillard. Then again, I might be wrong - it could be the flange diameter creating the creep.

Check for yourself...

When I build electric motor hub wheels where the customer says, "all elbows in please...", I'll chop off another .25mm of spoke length of the calculated result knowing that I no longer have to leave it in to accommodate outside spokes since they won't be present.

I do it just because I can...spoke machines give you that liberty.

(Electric motor flange thicknesses are often 3.4mm to 3.6mm...from what I've seen so far...)

=8-)