If your tax withholding with no change to your withholding rate would be $24,000, increasing your tax withholding by a total of $2,400 over the next 6 months would bring you to $26,400 of federal tax withholding which would satisfy the safe harbor of 110% of last year's tax withholding.
Don't forget to consider state taxes where you might have to do the same sort of thing.
