Often times I get readers who like one portion of the story and others readers who dislike that portion. I was wondering if there were any metrics or approach people use to determine what needs fixing and what doesn't.

Which one brings you closer to your intent or vision for the story?

Whether I'm getting feedback from other writers, editors, or straight-up readers, I always consider the feedback as a symptom of an issue. Very rarely will feedback ever give you the actual solution that you need. Your job (our job, as writers) is to look at these symptoms and diagnose what the actual illness is with that part of the story. From there, you can usually figure out a remedy that addresses the feedback in a way that works for you and the readers. This is especially the case when you get conflicting feedback.

It is impossible for everyone to like your story equally so you might get praise and criticism for the same thing. My rule of thumb is to take the feedback, go through the portion of the story it concerns, Chek whether someone else has pointed a similar issue and then... act as my gut dictates. It is my story after all. 

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I tell myself if three or more people comment on a common thing, it might be worth revisiting in more depth. But ultimately, don't do anything to your world/style that feels *wrong*.

None.

Or alternatively, the part you would personally enjoy fixing.

The thing is ... 

I find it unlikely that your readers will go back and re-read the chapters. To do that, you need to be really invested in the story, and not all people who already enjoy the story are that invested. I personally dislike rewrites as they often bring me out of the story I've already read through because, in time, I grew to accept what was wrong the story simply being this way. Every rewrite makes the feel I am not in the same story anymore...

I am certain it won't be the same for everyone, but I still think that most people simply accept your story for what it is, as long as they like it.

Only those who are personally invested, like co-authors, may actually want this thing perfected. 

TL;DR: Rewrite only if it brings you happiness 

*) Applies for serialized web novels posted on sites like this. Traditionally published novels were endlessly rewritten until they reached their final form you read. If you are aiming for traditional publishing, try to fix all the "mistakes" 

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J.J. Wrote: Whether I'm getting feedback from other writers, editors, or straight-up readers, I always consider the feedback as a symptom of an issue. Very rarely will feedback ever give you the actual solution that you need. Your job (our job, as writers) is to look at these symptoms and diagnose what the actual illness is with that part of the story. From there, you can usually figure out a remedy that addresses the feedback in a way that works for you and the readers. This is especially the case when you get conflicting feedback.

This is very true. I'm actually doing the same. If someone didn't like or didn't understand a scene, it doesn't mean that the scene must be erased or that it doesn't belong to the story. It just means that I failed to make it work, that it lacks something (further details, emotions, or whatever), and that I need to cut or add some sentences. 

I had a lot of comments from people that were confused because in my story at the beginning one character says "I'll tell X" and at the end, the reader learns that X actually was never aware of it, meaning that the character lied about telling X. I thought it was good for suspense but people felt like it was a plot hole, so I added little details throughout the book like "it's weird it feels like X is not really aware of..." in order to prepare my readers for the fact that it was a lie. So yeah now it's not so big of a surprise, but I don't get "I don't understand????" comments anymore and no one has complained it's obvious now so I guess problem fixed haha.

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Listen to feedback that expresses confusion, if you wish to make your story available to a wider public.
Listen to feedback that questions the intent of your story and characters, if you wish for your readers to understand what you wish to convey
Ignore feedback that questions motives you don't wish to clarify, or that questions why your character has specific undesirable traits (unless you don't actually want your character to have such traits)
In the end of the day, you have a story, and it can be told in a million different ways, find the ways which resonate with you, that bring the story closer to your ideal, and use reviews and critique as stepping stones to get there.
You will never be able to please everyone, but stories are not made to be pleasing, they are mirrors, worlds diluted into words made of mercury, and as long as your words reflect your world just the way you want them to, you have the best possible story.