true-count insurance

I'm hoping someone can help me determine how a true count of '+3' for insurance wagers was derived for hi-lo..

It would seem to me that a true-count of '+4' would be a garantee of the appropriate 10/'other' ratio... but I'm also able to take certain cards from the deck that yield a negative true count (aces with neutral cards) while still making buying insurance correct...

So I guess I'm asking what the logic is used for determining the best overall 'average' true-count...

Thanks in advance...

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