NightStalker you're right.
First, the concept of insurance and even money are mathematically identical.
When you play the insurance side bet when you have a blackjack, the name of the side bet changes from “insurance” to “even money”. Rose by any other name,...
The insurance is an independent side bet that has nothing to do with your hand. You’re not insuring your hand, but rather, you’re betting that there is or isn’t a ten in the hole, so you need to assess the probability of a ten being in the hole.
The reason for my “mispost” previously was because, as NightStalker said, the mix up between 3:2 vs 6:5 games.
If the insurance payout is 2:1, then you play the insurance side bet when the probability of a ten being in the hole is approximately 50%, which is when the true count is approximately 3 or so. At true count higher than 3, the chance of a ten being in the hole is greater than 50%, so the 2:1 payout is a great side bet.
In a game where the blackjack pays 6:5, if the dealer shows an ace while you have a blackjack, the insurance side bet payout change briefly during that moment. For a brief moment when the dealer shows an ace and you have a blackjack, the insurance pays 5:1 rather than 2:1. If the insurance side bet pays 5:1, then play the side bet when the probability of a ten being in the hole is 20% or greater. At what count is the probability of a ten being in the hole 20% or greater? When the count is negative. So,... basically, take even money at the 6:5 game.