On the RV-14, the two parts are called the S-602-1 Spinner Plate (which is the one on the aft of the propeller, with the flange open toward the engine) and the S-603 Front Spinner Bulkhead. I am referring to the aft one. The S-603 flange is prepunched and I plan to use the magnetic ball method from Section 5.18 for those holes. The challenge is the aft one, where the flange is not prepunched and the plans call for holes to be drilled where the longitudinal lines on the spinner cross the mark that the builder is supposed to make around the circumference of the flange of the spinner plate.
A translucent spinner would make that easy. An opaque one requires some way to locate the mark on the spinner plate flange behind the opaque spinner. Your method is one that I had brainstormed about. I can measure back from the web of the spinner plate to the edge of the spinner, then measure forward from there to where the mark is.
The alternative seems to be to pre-drill holes in the spinner plate flange as close to where the spinner is marked as I can, and then pre-drill the spinner plate before using the magnetic balls to match drill the spinner.
Of the two, I like your method better because any imprecision in my technique will result in the holes being slightly off the mark on the spinner plate, whereas imprecision in the magnetic ball method might result in the spinner being slightly off its pushed-onto-the-spinner-plate position.
Thanks for the input so far on this one. I'll wait a bit to see if other tips come in before I grab the drill and dive in.