you don't have to calculate it
just enter how much you actually received for each of the 1st two stimulus payments and TT will put the correct answer on Line 30.
the 2019 tax return has nothing to do with this. it is all about the income, marital status and children under the age of 17 on your 2020 tax return LESS what you ACTUALLY received for the 1st two payments.
why do you want to calculate this yourself? see page 58
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
