I have rarely been able to center and keep centered a side pull using the method you suggest. Maybe I'm a poor mechanic. Not.
An example for using a double washer is the Medici I am building up. The fork must have been designed for 27 inch wheels because 700c wheels will not work using a brake with 57 mm of reach and one serrated washer. I can, however, get this brake to work using a double washer. The double washer pushes the front brake out just enough that the pads will properly contact a 700c rim with the pads all the way down in the slots. As a bonus, it gets the cone flats out far enough that it clears the head set so I can use a cone wrench to center the caliper. And I don't have to use drop bolts front and rear.
But really, if a double washer allows one to use a wrench to center the caliper, what's the harm? It's much easier to use a cone wrench to center the caliper compared to removing the wheel every time you need to hold the caliper off center, tighten the bolt, then hope the caliper will be centered once you have tightened the bolt. And, if the caliper is not centered the first time, you have to repeat the process. atmo.
Jay Sexton Sebastopol, CA
I still do not see the purpose of the extra flat and serrated washer. If the idea is to shim the front brake out over the headset bottom cup so that it can accessed with a wrench, I understand it less. Every brake I have ever had that needs centering can be centered by holding it cocked a bit to one side and then tightening the rear bolt. If the brake ever would require a wrench to hold it, the wrench could be applied with the wheel off from the bottom.
I vote that the extra serrated washer is a spare and the flat washer is an aberration.
Garth Libre in Miami Fl USA