The way this works is that whenever you run out of bandwidth the system will order an amount to get you to 50 GB. This includes if you get a influx of viewers and go into the negative.
For example, let's say that you get a large rush of viewers right when
you are about to run out of bandwidth. The system will enter into a
"grace" period while it updates the analytics, and once over your
bandwidth has dropped to -3 GB due to the rush. At this point the
overage protection kicks in and orders 53 GB. This will bring your total
to 50 GB, as stated by your level of overage protection.