S&P 500 KISS system:
2000 SPX high 1553 SPX. 2002 SPX low, 775 and 768, which represented a 50% re-trace off the high from zero and a re-test respectively.
The 2007 high on the SPX was 1576, effectively a test of the 1553 SPX 2000 highs.
We just tested and breached the 2002 50% re-trace levels last fall. The re-test failed last week. Typically when that occurs price will continue to the .618 re-trace area, which is at 593 SPX.
There is little in the way of historic support in the range from 500-SPX to 741-775 SPX, so the .618 re-trace is the best hope for stopping the slide down the elevator shaft.