The foot may not contain the decimal beautify of the decimal system, but the 12 inches relies on the superhero abilities of the number 12. With no math at all, you can divide 12 into 2, 3, 4, and 6. As half- and quarter-inches are easily produced, you can divide by 12 and 24. That's an absolutely massive and flexible range.
Compare this to metric. The formal math for base-10 is easy, but it only divides by 2, 5, and 10. However, that doesn't mean that you can't divide by three. The designer could easily choose a scale which breaks across threes easily.
Because art deco must be manufactured, the designers aren't going to use crazy numbers that nobody can reproduce. Their work will be based on blueprints, which contain reproducible numbers. That means that the English units should break along major English dimensions, while metric should break along major metric dimensions. You should see the the footprint of the original measurement system in the relationships of the objects. You are more likely to fifths and tenths appear in metric while you are more likely to see thirds and quarters in English. Doing so tends to make the math easier.
The second thing to keep in mind is that the process of construction is inexact. There will be errors. Perfection is impossible. Architects and engineers know this. Their designs take this into account. Don't get too caught up in looking too closely at the numbers. If your numbers are all coming out too complicated, you likely haven't worked out the correct answer for your design. More often than not, when you hit the measurements right, you'll find simplicity.
Everything builds off the scale and the measurement system, so take the time to work them out.
Return to: Art Deco 101
Douglas Milewski is a fantasy writer who liked drafting class too much. In his recent artistic struggles to produce art deco for his own covers, he found no internet sites dedicated to the technical underpinnings of the art. Seeing a niche that needed filling, he has documented his hard learned experiences. He doesn't claim that he's right, and would very much appreciate it if someone more competent would save him from his own folly.