Perfect index for insurance

Hi there,

What is the perfect insurance hi-lo strategy number for 6D game? 217/72 (~3.01389)? The number 3.01389 means that for any 52 cards of the shoe, on average, high cards will be more then low cards by 3.01389. Middle cards are not tracked by hi-lo system. So, on average, we could suspect that there are 12 middle cards in every 52 cards. Thus, in a pack of 52 cards, high cards will be 21.506945 ((52-12)/2+3.01389/2). Four of five high cards will be ten valued cards. Thus, the number of tens will be 17.205556 (21.506945*4/5). But 17.205556/52 is less then 1/3. So, it looks like that the index 3.33(3) is the precise number.

It will be very kind if somebody throw some light on my ignorance.

Blackjack Rookie