The simplest way to forecast expected return of the market

Recently I saw an interview below on John Bogle asking his view on the current market, and I thought it was really an interesting and simple way of using it as a guide. John Bogle, who is the founder of the Vanguard Group, shared this method which he had been using for some time. You can find the full video here.
Essentially, it consists of 2 main concepts; of

Investment Returns + Speculative Returns

Dividend Yield + Earning Growth +/- Change in PE ratio

Investment Returns = Dividend Yield + Earning Growth

Speculative Returns is the change in emotion of people in the market on how much they are willing to pay. 

John Bogle also admitted that no one can predict the future, however he advised investors to stay on course, don't move money here and there, and keep investment simple. These are very good advice.