It's great to see you actively posting again.
Thanks. Not sure how long I'll continue though.
This will track nominal GDP if corporate profit share is a constant proportion of nominal GDP and the share which is listed is roughly unchanged (one reason why Buffett probably regards market cap to GDP as a valuation indicator, I think).
Yes Hussman has been explaining this a lot recently in his WMCs. Basically, valuation measures like CAPE are good, they have a correlation approaching 70-80% to 10y nominal total returns.
But if you account for variations in profit margin, the correlation is much much higher.
Turns out that "accounting for the profit margin" essentially gives you Price/Revenue (which I think is Price/Sales with a different name?) ratio.
Also turns out that Market Cap / GDP is essentially an almost perfect proxy for "total market Price/Sales". The other one he mentioned recently in this vein was Market Cap / Gross Value Added which supposedly has the highest correlation to future nominal total returns without any tweaking required.
(from http://hussmanfunds.com/wmc/wmc150601.htm)