Simply stated, the market has another 20% in it because it hasn't yet reached the extremes of valuation which we witnessed in 1999. That's not to say that it must reach those extremes, but it is possible.
At that time, total market cap to GDP came to 1.72, using the Wilshire 5000 as a proxy for the total market (times 1.2).
Since current dollar GDP is $17.5354 trillion, the current ratio is only 1.42, but the level of the Wilshire 5000 implied by a ratio of 1.72 would be about 20% higher than where it is today (total market cap of $30.1609 trillion or Wilshire 5000 25,134.07).
The Wilshire 5000 starts the day at 21,005.50.
Good luck!
