There are no chain values at all when horde hunting. You just have a 1/4096 chance with every Pokémon in the horde, in every horde, so it's essentially multiplying the odds by 5. There is no chain or anything you can do to make the odds higher, except for obtaining the shiny charm, which makes the odds 1/1365 per Pokémon in the horde.
Source: knowledge and experience.
