S&P 500 Target Estimates - 600, 500, 280?

Robert Shiller maintains a spreadsheet of S&P earnings and P/E ratios. If we look at values for the downturn in the early 1980s and relative to severe historical recessions, earnings of $40 and P/E multiples of around 7 can be seen on the charts (click to enlarge) above => 7x40 = 280 as a target of S&P 500? Other numbers could give... 10x50 = 500? 10x60 = 600... http://irrationalexuberance.com/shiller_downloads/ie_data.xls