Fly Depth Rules For a given speed of current flow, which sinking line or sink tip should we use? How long should it be? How much difference does a weighted vs. unweighted fly make? How should we adjust for slower or faster water conditions? How much difference does the type of floating head make (e.g. Skagit head vs. WindCutter)? How much difference does a density-compensated cheater make (inserted between the floating head and sink tip)? It is now possible to answer all these questions. Sinking line specifications currently provided by manufacturers (overall grain weight and sink rate in still water) are, by themselves, poor predictors of fly depth. In moving water, other factors such as line diameter, grain weight, and water velocity are extremely important. We introduce the concept of "rule number" to enable fly fishers to address these moving water factors easily, offer a pick your depth formula to help them determine the length of sink tip needed, provide a simple way to estimate water velocity, and graphically show hangdown profile and fly depth for a range of setups. Please enjoy and contribute to our forum on these topics (link below).