Keep in mind it will only monitor the computer it's installed on, if you have more than one computer you need to install it on all of them and add up the results.
I used Bitmeter after BT said I went over my 10gb monthly allowance ,but the readout was crazy it got so it was clocking up even when I disconnected my line and showing 200 mb two or three times a week when the pc wasn't being used I'm not sure if was because my pc is 64bit and Bitmter didn't like it or what