Er, I'll take a stab...
Fi can be like an internal compass at times. My emotional feeling can be very vague in the sense that it's so layered & nuanced it is often not clearly "happy" or "sad", but it's still pointing me in a general direction. However, I still have to map it out to get to the destination or conclusion (sometimes you go the wrong way at first...but you will navigate your emotional terrain with more confidence the more you do it).
This involves reason of course. A significant emotional feeling may remain vague, as often it's so broad it will be a trigger for forming many values (or emotional explorations), but now I will have some clear values defined from it. The more I explore the emotion, the more I will come to understand what I value. Then I consider what the value itself means, how it connects to other values, and in that process, my base principles become clearer, and I can identify them with broad ideas like "peace" and "honesty".
INFPs naturally spend a lot of time reasoning on why they feel something, what it means, and how it relates to themselves. Introspection is the key. It cannot happen overnight. You will probably have to search a lot before finding an answer within. Fi-doms may do it a bit faster because it's very natural to do it, but even we spend much time mulling something over. In all honestly, it's nothing close to this linear and it may occur in images or waves of atmosphere or random phrases popping into your head that represent the feeling behind an emotion, accompanied by a thorough examination and deconstruction of these feelings & emotions. That's why Fi is hard to define.
Try breaking down your Ni insights and reforming them by relating them to yourself. I'd also ask some ISFPs how they feel their dom-Fi works with their tert-Ni.
I think the greenlight wiki exercise below is a decent way to get the ball rolling. "Self-empathy" might be a good way for you to look at it. I think that Fi reasons more than this describes; but before Fi reasons, it identifies what an emotion means, or at least one of its possible meanings. Right now, you need to get to that point before you can form values.
From greenlight wiki:
To experience Introverted Feeling:
- As you come across the action of any mammal engaged in any activity (including humans), say to yourself, "He/she is feeling ______ because he is needing ______" and fill in the blanks. Guess the mammal's emotion as accurately as you can, by paying close attention to every detail of its behavior and trying to imagine what emotion that you might feel if you were that kind of mammal and acting that way. Guess the need by intuiting the inner calling of the animal that is emerging in the way it's responding to its environment, by recalling a similar need of your own. For example, if you see a Scotty dog sniffing around at a new suitcase, you might guess, "He is feeling apprehensive because he has a need to know he's safe." Or you might guess, "He is feeling curious because he has a need to learn all about the world around him." It depends unpredictably on exactly what you really observe. Key is to watch the mammal extremely closely, so your guess emerges spontaneously from empathizing, and not, say, by consciously reasoning on the basis of something you've read. Your guess must come from the fact that you yourself genuinely feel it. It must come from the heart.
- Try the same exercise on yourself at odd moments: self-empathy. Simply monitor how much you like or dislike something, and what in your nature is being fulfilled or frustrated to cause that feeling of like or dislike. Note that attending to your emotion alone is not enough; you must trace the emotion back to a need that is being fulfilled or frustrated. However, if you're having trouble with this, you might try just consciously noting your emotion for a while, as a starter exercise.