Yeah - that's the way I would handle telling her.
Well, I wish you the best. Hope all goes well.
Don't tell her unless you can live on your own and support yourself.
My mom is rather religious, it's a fluid scale of religiosity we're working with, so I'm not sure how the cases compare. I told my mom that I don't believe in any god, but I haven't gone into just how averse to religion I am. When I told her, she cried and told me I'm going to hell. My lack of belief has caused a lot of friction in both my immediate and extended families.
Here's where the difference comes in. I have a few family members who don't care that I don't believe and so I get a sort of safety net.
I'm not saying that you should necessarily just alienate your family if you still need them when I say this, people should love you for who you are, not what you believe. Once you can support yourself though and you don't need to rely on anyone, I recommend being your own person. Lying about who you are and trying to live a double life is harmful and I wish the very best for you as you decide about this.
EEK! I no longer talk to either of my parents, partly because of their trying to force religion or religious beliefs on me. When we did talk, though, I found that writing letters was best. I could take my time and re-read and re-write them. I'm not forced to watch their faces when they read them..it could hurt me or cause an immediate argument. This gives us both time to think about the letter before discussing it.
In the Bible there are two stories, one about a tax collector and the other about a prostitute...Jesus forgave and was accepting and welcoming of both. Maybe remind her of these stories in your letter. But, I bet she'd love you no matter what. I think your biggest problem will be defending yourself politely against her constant 'witnessing' to you! I think this 'witnessing' is rude and have cut off most of my family because of it. You'll need to find a way to handle it.
The decision to wait or tell her right away would be entirely base on how well you know your mother. If she's going to flip out, it is safer to wait. I was able to talk to my mother while I still lived with her and our relationship did not change. The thing you'll have to get used to is that she is going to try and convert you back. Be firm and if she tries to talk about it let her know its not open for discussion. If she tries to send you off to a camp or something firmly tell her that you're not going (and stick with it). Keep any discussion calm and if things get emotional, take a walk and let emotions cool down (if there's yelling, that's a good sign to postpone the discussion).
If you think that she really won't be able to handle it or that she'll go to extreme lengths to try and convert you back, wait until you're out of the house.
I was lucky it worked out between myself and my parents but they still send me God stories and ask if I'll go to church. Something you can't avoid if you want to stay close with her.