I believe you also have the option to take a deduction against income in the year the repayment is made. This might be more beneficial, especially if you repaid in the year you turned 70.5 and could use the deduction to offset an RMD and maybe even an additional IRA withdrawal. Since this would lower your IRA balance, it would reduce RMD's going forward, as well.
So far as the tax credit goes, I think the easiest way to calculate it, would be to set your SS to zero each year in your tax software, and enter the difference in tax owed into a spreadsheet, or other ledger. This would automatically pick up any "tax torpedoes" that might have occurred. Then just sum the numbers when you repay to determine the credit, which you can then compare with the savings from taking the deduction.