Prudence is the standard by which the IRS and financial institutions and most other observers of the nonprofit sector measure the performance of a nonprofit board. Prudent behavior is best measured by comparison answering the question: "what would the majority of other boards or businesses do if faced with similar facts, resources and decisions".
Hence, how prudent has the Board of the Asian Art Museum of San Francisco been while it faced significant financial challenges. You can be the judge based on the information provided in this Wall Street Journal.
I for one am not sure I have all the qualifications to judge but I do believe the Museum offers some good insights about what exactly prudence may look like.