1. With a small, sharp knife, cut off both ends of grapefruit. Following curve of fruit with knife, slice off peel and white pith. Cut between inner membranes and fruit to release segments; put into a medium bowl.
2. Squeeze the membranes into a strainer over a measuring cup to extract the juice and to remove any seeds. You need 1/4 cup juice; add a little lemon juice, lime juice, or Champagne vinegar to make up the difference. Stir in olive oil and set vinaigrette aside.
3. In a large bowl, combine the lettuce and fresh herbs. Pour the vinaigrette over the lettuce mixture and generously season with salt and pepper to taste. Carefully lift the lettuce and herbs out of the bowl and arrange them on a serving platter.
4. Add the avocado slices to the bowl of grapefruit and pour remaining vinaigrette from lettuce bowl over them. Toss gently just until coated, then scatter over the salad.
5. Sprinkle the shrimp over the salad and drizzle any vinaigrette remaining in the grapefruit bowl over the shrimp. Serve immediately.
Note: Nutritional analysis is per serving.