On your 2019 return, this would be line 18 (Net Amount of Tax) minus the sum of lines 23 - 25.
Estimated tax payments, income tax withheld and extension payments are not considered tax credits.
For line 3 of your 2020 Form 760c, you need to enter the 2019 Income Tax Liability After Spouse Tax Adjustment and Tax Credits.
If you'll look at your 2019 Form 760c, the Spouse Tax Adjustment is shown on line 17.
The Net Amount of Tax, after the Spouse Tax Adjustment, is shown on line 18.
