Maybe this helps...
When I use Ne to figure something out or make an association aka a gut feeling, I'll have this 'ohhh oh ohh!' feeling and it'll physically be felt in my head. I'll be all excited as well.
Whereas with Fi, I'll have two choices in front of me, close my eyes and think of each choice seperately, without analyzing it. The one that gives me the warm, fuzzy, true feeling, that literally warms my heart, is the one that's right for me. That's Fi. And for me it is felt near the heart. And upon reaching that decision, I'll feel calm and at peace, as well as happy, possibly even filled with bliss.
As for a definition of Fi in SPs, I'd say, it's what helps you decide whether you like a piece of art or not, whether you're on the right track when creating art. The thing that tells you that in a song, that one high note was exactly placed at the right place, to give you those extra shivers running down your back. It makes you realize that something is just right for you, resonates with you and is in sync with you, whehter this be a piece of art, a song, a decision, the way a home has been furnished, anything really