After giving it some thought, Thomas' method is probably the easiest way to do a geometric series (if you can remember the standard f/stops on your lenses). His method has a bit coarser interval than mine, but we are basically doing the same thing and probably would be very close on a printing time. Mechanically, Thomas' way is simpler because he is giving really only one continuous exposure and doesn't have to reset the timer. This also eliminates any intermittency effect, or as Beth has noted, cumulative timer error which could be happening with the method I suggested.
I'm a convert Thomas. And I'll recommend your method from now on.