For a better (in my opinion, of course) tamarind margarita, try Danny Meyer's Tabla restuarant. Don't know proportions, but I believe they use gold tequila, cointreau, tamarind juice, and orange juice (maybe lime juice too?). Bartender told me they extract the juice from the tamarind fruit. It's my favorite margarita in the city... it's $$ at $12, but hey, I'm worth it