Think of it as fair value.
Another way to get there:
GDP(63) = S&P fair value
$22.061 trillion x 63 = 1390
That formula worked for most of the post-war up until the Trump era. The fancy regression analysis done by the geeks always came up pretty close to the same result, but not lately.
Still another way to look at it:
S&P 500 4230 / GDP 22.061 = 192.
That's an elevated ratio which was common before 1929, but we've never seen such levels in the post-war.
Sum ting funny goin' on.
