I think being safe is the most important thing, so waiting till your out of the house if it isn't long is a good idea IMO.
One bit of advice, don't apologize when you come out. Do it from a position of strength, don't act like you owe anyone anything or did something wrong. After all, you don't. I have seen parents undermine their children much more often when they are telling them something big, as if they had done something wrong. You haven't done anything wrong, but if it comes across like that it can be far too easy to fall into familiar patterns, after all most parents are pretty used to dealing with their kids doing something "wrong" and tend to have knee jerk reactions on how to deal with that. If they never fall into that pattern they will do more actual thought on the issue.
I am not saying you should argue with them either though, state how it is, don't invite a debate. If your parents think this is an argument of some sort they will likely not really listen to the words you say. I have yet to meet a parent that didn't think they know better than their child at least most of the time. If you can remain calm and collected while you inform them about all this it will help tremendously, which is why I think coming out letters are many time better than coming out in person (I have done both, depending on the person).
Good luck for when you do tell them, they might react better than you think!
