From what you said if line 8c is just reporting the maximum credit that is allowed. I am not able to make sense of line 8d {8a-(8b+c)} based on return should be zero. My return actually shows a number 534(I am not how they came up with that number).
I have a schedule H from third party payroll service( sure payroll) which i use to compare with TurboTax there seems to be a discrepancy in lines 8c and 8d even though at the end credit is correctly not applied.
