Still not optimal.
The Siege type of battles includes sallying too, and its more than likely that you tend to kill more than 70% of the besieging force when sallying.

Also, using the <30% makes all cavalry tactics count as more dreadful, as you don't tend to lose a lot if you charge properly, while with infantry you do.
Using massive missiles or projectiles would also result in more dread.

To be honest though, I don't really get what is meant with BattleDread in the first place. Sure, the various traits seem to point towards "wanting to win a battle more than doing so honorably" or "Mutilates the current foe to inspire fear in the next" or "Pikes a few heads" but those are subjective and its not like you have an option to mutilate foes during battle, or using dirty tricks.
I've been thinking about linking BattleDread and BattleChivalry to releasing/executing prisoners, although those already have their own traits.

Similarly, BattleChiv requires you to not hunt down/kill too many enemies.
That means you have to give them opportunity to rally and fight again, which may or may not be intentional either, as an infantry heavy army cannot keep up with the enemy, making him more likely to rally and similarly such an army would not be able to hunt down routers.
Also, It makes little sense that winning a heroic victory while being outnumbered should necessarily give you double the dread points. For all I know, fighting against the odds would certainly be more chivalrous than bringing twice the number of troops your enemy has.

Conclusion: take both traits out completely or just don't bother. They don't make much sense anyway.