Here is a daily chart of the cash S&P 500 covering the past year's price action.
I think the drop from the April 2 top at 1422 is a reaction within an ongoing bull market. To estimate how far it might carry I like to look at the size of previous reactions within the trend and use these as my measuring stick.
Since the October 4 low at 1075 there have been two obvious breaks. The first one ended on November 25 and carried the S&P down 134 points. A drop of that size from 1422 would put the S&P at 1288 (large blue dash rectangles). The second break occurred in December of 2011 and carried the S&P down 64 points. A drop that size from 1422 would carry the S&P to 1358 (smaller blue dash rectangles).
The green up trend line I have drawn right now stands near 1355. The last low of significance was the March 6 low at 1340.
I think the best guess now is that the S&P will be supported somewhere in the 1340-58 zone (purple blue oval). The subsequent rally should carry the market above 1422.
If support in the zone delineated by the purple oval should fail then I think the next step down will be to support determined by last years 134 point break in November. This is 1288 and coincides with the October 2011 top at 1292 which should be support in its own right.