I am not so sure about dead on. I had the same situation except that the deer was about 15 feet. The slug went over its back. It stood there, I aimed low and killed it. At 20 yards you may be able to hold dead on, but who really knows.

Sorry folks, but no one on here can tell you how your set up is going to perform. They can only provide "educated" guesses. I suggest test it yourself.