If that means a midpoint of 8,500 and the overcorrection is 20%, then the momentary low point could be around 6,800, which at least suggests that the gap between my two red lines is approximately correct.
Friday's lowest point during the day was 7,773.71, still 10% away from the theorised minimum; and the Dow closed at 8,451.19. Yesterday it remained above the latter figure throughout, and rose to 9,387.61.
In short, Grantham must be reading this as a bear market rally, and it's not very silly to think that the Dow could come back to 7,000 at some point.
Good luck to the day traders, I haven't the nerve and speed to try to make a fortune on the bucking-bronco stage of the market.