Those who don't have an unlimited Internet connection has to keep track of their bandwidth to prevent over usage. A lot of people use desktop bandwidth monitoring softwares, but as you might have already found out, they are far from accurate. So you always have to rely on the data provided by your ISP. For Firefox users here are two useful addons that will enable you to keep track of your Internet usage from within Firefox.
Net Usage Item supports 38 Australian ISPs and additional ISP packs are available that adds support for a couple of ISPs from Belgium, New Zealand, Canada, Ukraine and Indian (Reliance).
For Indian BSNL Dataone and MTNL TriBand users, there is another addon - DataFox. DataFox fetches net usage information from the respective ISP's web pages and displays it as a small bar in Firefox's status bar. Additional information is available in the tooltip. Statusbar and tooltip strings are completely customizable.
The progress bar gives you an idea of the amount of bandwidth from your monthly limit already consumed. Different color codes will alert you whenever you over-limit for a particular day. This will help keep your bandwidth usage in check.