Hey there Migs:
As is often said " it's all relative". My droppers are usuall very dependant on dept of water and current speed. It seeems however that I use 18" for most of MY situations. The dropper cannot be of a weight that would cause the primary dry to sink. I usually fish with small "stuff" so the ratio of dry to dropper is critical but then I expect to entice a bite to either presentation. If you're using the surface fly as an indicator ONLY, it's size matters less as long as it floats with dropper attached.
Great article by the way.

Mark

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I'd rather be in Wyoming!