you are welcome to work through the Recovery Credit worksheet on page 58 of the attached.
if you look at lines 17 (1st stimulus) and 20 (2nd stimulus) , it says 'if less than zero, you don't have to pay back the difference'.
in other words, Line 30 of Form 1040 can never be negative
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
is that what you are looking for?
