There are five decks remaining in the normal method of estimating remaining decks. The Custom TC Calculation doesn't use remaining decks. It uses remaining cards along with a table of multipliers or divisors for the purposes of calculating TCs. This is highly flexible, and therefore impossible for the program to know that you are thinking of this as six decks instead of five decks. It just uses the table for TC calcs. So, the TC calc should be as you wish. But, it will not display remaining decks as you think of them as it has no way to divine this.

If you wish, you can turn off Custom TC, change the TC Res and Last Deck Res, and turn it back on. These options control how remaining decks is displayed, even when Custom TC is on. When set to Full deck, remaining decks is displayed as an integer. When set to exact, it is displayed as a decimal. When set to Half or Qtr deck, it is displayed with a fraction.