The tippet length you should use depends on factors such as fish spookiness, fly size, and specific fishing situations. As a general guideline, consider matching the tippet diameter to the size of your fly and the behavior of the fish you are targeting. For example, when fishing in clear water with easily spooked fish, using a longer tippet (around 3 feet) with a smaller diameter can increase your chances of success. However, in situations where fish are less wary or when using larger flies, a shorter tippet (around 18 inches) with a slightly thicker diameter may be more appropriate. Remember to adjust your tippet length based on on-stream observations and personal preference for optimal results.