How would one translate "a fine line"?
I find it hard to know what you can and can't translate literally.
"A fine line" means "very little difference." How to translate a phrase depends on the language being translated into--if there's the exact phrase in the language, use it. Otherwise look for an equivalent phrase. Lacking that, do your best to paraphrase the meaning. Naturally, this is easier when you know both languages very well, so reading and practice are vital.
I don't know what you're translating, but maybe, "Ekzistas maldika lineo." (There exists/is a thin line) would be appropriate? It is difficult to know what will be understood across language barriers. English speakers would probably understand the phrase, but others might be confused.