1/2/20 Update: The IRS has not issued a new W-4P withholding form for 2020 yet. Keep checking the IRS website for further updates.
Use the IRS Tax Withholding Estimator to calculate your withholding. It works really well.
Note: ACERA does not allow you to withhold a flat amount with no allowances. This means you MUST enter a number in the Allowances box (box 2).