8-Deck Shoe Game (~69% lousy penetration)

I use Hi-Opt II + ASC. Yesterday, I had a chance to hit or double 8 against 5 at +4 TC (I use 'flooring' when calculating for TC; it was somewhere between +4 and +5). I was so eager to double, but the index was doubling at +6 TC or above. The dealer showed his down card, which was a 10, and busted so I won.

I know I followed the index and I shouldn't regret about it - but was wondering for the reason we double 8 against 5.

Are we doubling in hope of getting 18 or 19, or in hope of dealer to bust?