Yeah, last think you need is for you to sound like the stalker type in your letter.
Dont rave too much about how much you love her, just try to keep it sweet.
I wouldent worrk about it too much. There is no fine line for how her pairents will interprit your letter, and you wont be able to get it perfect word for word.
I guess its like an exam, you can spend ages going for an A+, but a pass will still do. Im sure her pairents wont mind that much, as long as you sound 'nice enough'.

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