In emergency stopping situations, only the front brake matters. If you are using your front brake to maximum efficiency, there won't be enough weight on the rear wheel to make any difference anyway.

You should practice in a parking lot or something until you can judge how much to pull your front brake so you're stopping as fast as possible without flipping over the front wheel.

Also, you'll get more stopping power if you can shift your weight back. If you have the reach for it and practice it, you can even drop your butt off the back of the seat and drop your weight way down. I wouldn't suggest trying that until you're pretty good at modulating your brakes though.