Based on our experiments and simulations, we have found that every uniformly constructed sinking line has its own characteristic "rule number." This rule number is the number of feet of sinking line required to deliver a small, unweighted fly to a hangdown depth of 1 foot in 3 mph water.
Different sinking lines may have very different rule numbers. For example, one intermediate sink tip we tested is rule 16, which means that 16 feet of this line delivers a fly to a depth of 1 foot in 3 mph water. The type 3 166 grain 15' tip we tested is rule 10. Several commonly used sinking lines are approximately rule 6: LC13 (Cortland), T14 (Rio), Express 14+ (Scientific Anglers), and CCT20330 (Airflo).
We calculate a line's rule number by simulation of the principles of fluid dynamics applied to an otherwise "standard" setup that consists of a 26 foot 650 grain Skagit floating head, 3 foot 15 lb leader, and a small unweighted fly in 50 degree Fahrenheit fresh water. Given this standard setup, rule number is determined by line diameter and density, or, equivalently, by line diameter and grain weight (grains / ft). We have also done multiple experiments to validate these results.
Rule numbers for fly lines we have in our possession, have measured, and have specifically tested are shown below. This posting may be updated from time to time based on new information or on our testing of additional lines.
