Here is a weekly chart of the Spiders. (Multiply by 10 to get the cash S&P.)
The art of forecasting amounts to choosing an appropriate precedent for the current market condition. In the case of the Spiders I think the March 14 low at 136.75 was a low comparable in importance to the lows of August 2004, April 2005, October 2005, and June 2006.
On the chart above I have marked the percentage increase in the Spiders after each of these lows. I think it is reasonable to expect the advance from the March 14 low to be comparable to the advances from the preceeding four lows in the bull market. On this basis one can project targets at levels ranging from 150 to 163. Throwing out the lowest and the highest of these targets and averaging the remaining two gives an ideal target of 156.72. This is just a shade over the 2000 bull market top at 155.70.