By Daniel Bases and Rodrigo Campos NEW YORK (Reuters) – Measuring the point at which investors have exhausted their selling in a market downturn is an inexact science at best, and at its worst akin to sticking a finger in the air to judge shifting winds. By some measures, the near 10 percent decline in the Standard & Poor’s 500 stock index from a record high last month may already have flushed out much of the speculative money that helped propel a bull market in stocks beyond the average length. Investors look for a number of signs to determine whether a correction has run its course. …