Its not necessarily the number of the IBU as where it comes from. Getting 100 IBU from a 60 minute addition produces an entirely different level of bitterness than one done with only late additions.
Its just a number we can use to gauge how it might turn out. With that said, 1:1 works for me. Again, that is because I have a pretty consistent system of where those IBUs come from in terms of kettle additions.