Just look at the different web sites on the internet that sell fish. I guaratee you, if you want me to send them to you, I will. Some say 50. Some say 75. Some say 100. One book I have doesn't go so much into gallons but rather the length of the tank--six feet for optimum health, but four feet a bare minimum.
Having said that, it is a scientific fact that nearly all tangs are open ocean, shoaling fish that do well with lots of swimming room. However, in my humble opinion, cleanliness, strong water currents and a diet rich in greens and veggies are key factors in keeping a tang healthy. I bought a system that has been running for several years. It's a 55 that has a yellow in it, and he's okay, FOR NOW. However, I'm going to trade him in as he starts to grow. There will be people who say YES, and some who say NO, but the bottom line is that it's your tank, and you have decide. If he starts to outgrow his surroundings, be willing to trade him in or donate him to a larger aquarium.
Just my 2 cents.