I think, that to explain perfect insurance with dealer ace up - all it means is that there is confirmation that there is a saturation of faces sufficient to give dealer a blackjack a minimum of 33.3% of the time. The gain is the differential between perfect insurance and a potential shortfall when true count exceeds 3.0 Or 3.4 for hi lo halves respectively.
In other words, perfect insurance as a concept simply confirms that insurance is not necessarily a good bet, when true is around index, and that there is a surplus of aces, or for that matter, 9's when playing halves. If the count is a monster, it probably doesn't matter much, since, regardless of ace or 9 surplus, insurance is still likely to be a winning proposition.
There are other side counts, limited only by imagination, that can help with insurance calculation. A couple would be the Tarzan approach, as well as 3 type scenarios where attention is paid to intermediate groupings. The logical extension of a 3ish approach, for example, would be that insurance at 3.0 or 3.4 as mentioned above could be tightened or loosened depending on a deficit or surplus of intermediate cards.
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