Place the tamarind pulp in a large bowl and pour the boiling water over it. Add the sugar and mash the pulp with a fork until blended. Set aside to cool to room temperature. Force the mixture through a sieve, pressing down hard on the solids, and discard the solids (seeds and tough fibers). Add 5 or 6 cups of cold water and taste for sugar balance. Add more water or sugar as desired, then serve in a tall glass over ice cubes, garnished with an orange segment.