HiLo doesn't have a perfect insurance index
A perfect insurance index would tell you when the ratio of non-X's to X's in the remaining pack is exactly 2:1. Thus, to have a perfect insurance index, a count system has to count all X's in one group, and all non-X's in the other group.
One such unbalanced, running count, perfect insurance count begins with an initial running count (IRC) of -4*(number of decks) and counts all X's as -2 and all non-X's as +1. When the PIC is zero, insurance is an even bet; when the PIC is positive, insurance is +EV; and when the PIC is negative, insurance is -EV.
For example, say you're playing a heads-up SD game. On the first round after the shuffle, you're dealt 5-6 vs. the dealer's A. Now, the PIC is -4+3 = -1, so DON'T take insurance. However, if you happened to peek the bottom card of the deck when the dealer offered you the cut, and you saw that is was a 7, NOW the PIC is exactly zero, so insurance is a coin flip. Furthermore, if you ALSO managed to see that the burn card was a 2, NOW the PIC is +1, and you SHOULD take insurance.
Now HiLo does not have a perfect insurance index because of two shortcomings: first, it lumps A's in with the X's; and second, it doesn't count the 7's, 8's, and 9's. Furthermore, the calculation you outlined in your post is flawed, because you assumed that the neutral cards ALWAYS represent (12/52)nds of the undealt pack, but this is not the case.
Hope this helps!