First of all, know that this change in percentage tax rate is for the ATC for NON-VAT personnel:
If you are eligible for the 1% percentage tax rate, this is how you compute:
In your 2551Q filings for 2020 Q3 (due on October 2020) and 2020 Q4 (due on January 2021), choose View Form. If you have amendments, make sure to use and refer to the latest amendment only.
Once there, look for the values tagged against tax code PT010 in Page 2 of the Form.
To compute for what you SHOULD HAVE paid, sum up the taxable amounts tagged against PT010 in your forms. Multiply it by 0.01 or 1%. This is how much you should have paid as per the retroactive rules of CREATE.
Now, deduct this amount (the 1% amount) from the Total of Tax Dues tagged against PT010 in your forms.
If you submitted your 2020 Q3 and 2020 Q4 Quarterly percentage taxes through Taxumo, the appropriate field will be filled out properly. Otherwise, you may have to compute this based on the said filings.