The area in red is a future projection of the declines that have not yet happened.
100 is the baseline, the correction could also exceed the baseline on the way down. The current value is 115-120 or so (Extrapolated to 2012 from the chart), so I am guessing a 15% decline nationally from here and probably 20-25% decline in super-frothy areas like Southern California.
This is of course merely my opinion!