What is the exact fresh-to-dried conversion ratio for Mediterranean herbs?

Use 1 teaspoon of dried leaf for every 1 tablespoon (3 teaspoons) of fresh chopped leaf—a 1:3 ratio—because dehydration removes ~75% water weight and concentrates the oil by volume.

Why do woody herbs like rosemary and thyme dry better than soft herbs like basil?

Evolutionarily, rosemary, thyme, and oregano evolved tough, waxy cuticles and recessed oil glands to survive dry Mediterranean summers, which keeps their essential oils locked inside when air-dried. Soft-leaved basil and cilantro have thin cell walls that vent their top notes during drying.