Deductions & credits

I have had a longstanding question about foreign tax carryover in TT. In some years I have not used all the foreign taxes paid for that year as a credit and other years the opposite, I got a credit greater than foreign taxes (FT) paid that year.  The question relates to the running "carryover" chart. In i.e. year 2016, I paid FT of 318, had none disallowed, and got credit for only 91 that year. I would assume the amount utilized is 91 and the carryover is +227. However in 2017, I paid 521 in FT, had none disallowed and received a TT foreign tax credit of 613. Is the amount utilized that year 521 or 613.  If the latter, than is the carryover -92?  When I look at the TT carryover chart in the last column, is that supposed to be a series of + and _ numbers depending on what happens a given year or is it a running total, taking the recent year into account. When I look at the TT carryover tally over the 10 years I have used it, it seems incorrect and does not seem to take into account the years that I got a credit that was greater than the FT paid that year.

 

Thank you in advance for any answers to this.