"What is the benefit of downvoting a post?"

Rewards are reallocated to other payouts

"What qualifies a post to be downvoted in your opinion?"

My opinion as to the value that a post brings to Hive. I think you will find that my opinion is rather skeptical on this matter, although you probably don't want to hear it. I find that most content is a dime a dozen and doesn't draw a large audience. I don't have any disagreement with small payouts but once they get to be $10s or $100s, I want to see something compelling that it gives back. Most do not.

"And how do you decide the percentage that you are going to downvote?"

Usually a modest amount relative to what is being paid. If others want to downvote on top that is fine, and if it results in a bigger reduction, that is a sort of consensus. But usually when I downvote on my own the effect is just to trim the rewards modestly but still pay out quite a lot.