I thought the KO book suggested an insurance index of Pivot � 1 for all games ?
In the book the pivot is always +4 and the insurance is always +3.
In thinking of KO indices, it is better to think in terms of the Pivot. This is invariant. Then anyone can convert that to their own indices given their IRC.
I'm a bit confused about what you are trying to say here. The pivot index is invariant among the different number of decks only if you use the IRCs given in the book. The differences between the indices and the Pivot also change based on the number of decks.