Actually, the point of using more than minimal force is if she resists its effects.
Since its force 1, she just needs one success and she doesn't have any compulsion on her.
also you still take at least 2 drain, even if the value is negative. So.. I think influnce is -1, so you can cast at a force 3, with no drain problems.
Force also affects a few other things, I believe distance. More hits makes the compulsion last longer too If i recall. I'll look it up here soon.