Be extremely careful, it only takes one time to get severe chemical burns and you can lose a chunk of hair permanently from damage to the scalp. It sounds alarmist, but I've seen it happen too many times. It's really harmful stuff and just because you've done it so long and not had a problem doesn't mean you will always get away with it.
I have very, very dark brown hair too and I can get it to lightish yellow with 30vol, and you wouldn't even need to go that light, you just have to make sure you're working in a warm environment. I would also suggest bleaching, with a good bleach powder and then toning with a direct dye rather than using 40vol with a tint. It'll be safer in the long run.