You're on the right track here Mr Flip. Its good that she even wrote you a letter establishing communication again. With that small crack in the door you can do much good with this.
Bonafide gives good advice on keeping it light, I'd agree with that, try and keep her in her true personality. She showed that personality at the beginning of her letter then switched to her cult personality to "witness" to you. So I'd recommend trying to get her back into that true personality as much as possible. Perhaps reminisce about some childhood funny she's forgotten or some family holiday you've all fond memories of. (as long as no cult experience is mentioned).
Then she might be reminded that you were a father to her in many wonderful and diverse ways, not just the one that taught her "the truth".
I'm doing this with my dad right now. He's a very hardcore Jdub and has a core of steel running through him whenever the "Truth" is mentioned. You can almost see the switch! Yet when I remind him of the past and his navy days or when my siblings and I were little, he switches back to the kind loving gentle man I remember as my dad. The more I keep him in this personality, the more him and I get along.
My next plan of attack is gonna be "if you wasn't a witness, what would you be doing right now?" I'll bide my time for this question, but once he's in the good personality mode, I might get away with it.
Good luck with your daughter Mr Flip, she sounds a lovely girl and definately worth every effort to get back. Please let us know how things go won't you? x