How much sink tip do you need to deliver your fly to a desired hangdown depth? The calculation is simple based on rule number
and water velocity
Sink Tip Length (ft) = Desired Fly Depth (ft) x Water Velocity (mph) x Rule Number / 3
For example, suppose you are using rule 9 sinking line. You are fishing in 2 mph water and would like to get your fly to a depth of 2.5 ft. According to this formula, you would need to attach 2.5 x 2 x 9 / 3 = 15 ft of this sink tip to your floating head.
Note that this formula is the fly depth formula
, but just turned around. So it has the same accuracy: for 3 mph water, it is virtually exact by definition. Over a wide range other practical fishing conditions (e.g. water velocities from 1 to 5 mph), this formula yields estimates that are accurate to within about 10% of our more-precise simulation-based calculations