Adjusting leader and tippet lengths based on on-stream observations is an important aspect of fly fishing. Start by observing the behavior of the fish and the conditions of the water. If you notice fish are spooked or not responding to your fly, consider lengthening your leader and tippet to achieve a more delicate presentation. Conversely, if you find it difficult to cast accurately or the wind is causing issues, try shortening your leader and tippet for more power and control. Remember that personal preference and fishing style also play a role in determining leader and tippet length, so don't hesitate to experiment and adapt to find what works best for you.