Keep in mind that the law of large numbers has been systematically making the ratio less sensitive, and that the GNPs measurement has changed over time. The 75% cutoff may need to be lower.
Over the next 6 months GNP is projected to decline. For the trend to continue, the decline in stock price would actually need to accelerate.
The graph suggests a buy point at 50%, or less (35% over WWII). To get there the 'average' stock price needs to fall at least 62% [(50-130)/130] from the average 'peak'. Normal curve tails suggests there were will be some big winners & losers, & a way to quantify how many. Bear Sterns, Lehmans, etc. were losers, the equivalent offseting winners are still something of a mystery.
A laymans look would suggest that except for a very few stocks, its still too early to buy.
SD