For San Francisco-based billionaire Cari Tuna, the most important decision a philanthropist can make is deciding what cause to support . Starting Tuesday, the organization she helped found will offer help to other major donors making those choices.

Tuna, 40, and her husband, Dustin Moskovitz, 41, first seriously started in philanthropy in 2010, when Tuna quit her job as a journalist and set out to give away most of their wealth , which Moskovitz initially made as a cofounder of Facebook. But as she spoke with people in the philanthropic ecosystem, Tuna said she found few others who took a rigorous approach to the question that drove her and her husband: How do you use your giving to do the most good for the most people?

Instead, she was often advised to follow her passions, to gi

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