suggest walking your math through the official IRS worksheet; please post back if you think it is still in error.
The $970.60 gets substracted from the eligibility of each payment and the eligibility of the two payments is assessed separately; you appear to be combining everything.
see page 58 and then see lines 14 and 18
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
