Just go to the panel > status> Bandwidth usage and set up bandwidth throttling for the site by clicking on Disabled beside the site.
It also sets up a directory that it gets redirected to where you can put up a page that shows if you go above the bandwidth.
This won't garauntee that you won't go over but will limit it.
Bandwidth usage is updated every 24 ours anyway so it should be possible to see if you will go over and it may be worth upgrading if you will be going over a lot.
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