To calculate any usage, you will need an application that polls the interface counters, stores the results and allows you to access these results later.
Concord's eHealth is probably the best known, but is very expensive.
MRTG can be configured to give bit counts instead of rates (bps), but you have to know what you are doing. MRTG is free.
For choosing the capacity of a link, this is very difficult these days because of p2p.
Back in the "old days" i.e. 2 years ago, we would look at 5 minute average bps scores, and divide the peak observation by 0.75.
However, what you find these days is that you can double the capacity of a link, and it will fill up straight away. All that happens is that someone downloading music will notice their performance has improved.
You may choose to police or traffic shape the interface, and look at how many discards are occurring, and use this as a guide.
Hope this helps.
Regards.
