Approach for forecasting extent of rally?

I have seen articles over the years about technical approaches ('rules-of-thumb?') to extrapolate from prior price patterns and estimate the likely extent of a rally. With the S&P 500 in consolidation following the 2000 high at about 1500, and with a second top at about 1500 in 2007, that index is now nearly double that level – near 3,000. Can anyone remind me of the approach I have seen previously that estimates where the top may lie? Thanks!