With the S&P up 75% from its March 9, 2009 market bottom, it's smart to take a step back from individual stock valuations to look at the market as a whole. A good way to do that is with Robert Shiller's S&P 500 P/E data, which he used to predict the internet bubble of 2000 (weeks before it popped) in his book Irrational Exuberance. His excel spreadsheet is regularly updated at http://www.econ.yale.edu/~shiller/data/ie_data.xls. His methodology is simple. He compares the current price of the S&P 500 to the average index earnings per share of the composite companies over the past ten years. The resulting product is a chart of how the P/E ratio has fluctuated since 1880…