Why do you think the historical value is more valid than the current trend? Is there some dynamic in the market that keeps the p/e ratio above 15, for example?

I would think to calculate a "good" ratio you would need to compare it to returns from property and other investments?

How do you decide that stocks are P/E is too high now as opposed to it being too low in the past. The us equity market has historically had much higher returns than the rest of the world. There's no reason to believe that could continue forever. At some point world markets will reach an equilibrium.

If that 16 was a useful number, you could just sell when prices got to 17 and buy when they got to 15.