I think it is obvious.

You sell 5 plans, 5 GB b/w each then the total allocated is 25 GB.
But they only use 10 GB.

ratio = 10 / 25 = 0.4 (40%)

* If you think your audience affects this ratio (e.g.: game sites hosting), please mention it.

* If you are giving unlimited/un[whatever] bandwith usage plans, please don't include them in your calculations. The ratio will always be 0% for these plans.

Thanks :)