We use a couple of the Mifi devices from Millenicom to provide us with up to 40GB a month. For accurate tracking of usage the only 100% way to do this is with a router that will track usage for the Mifi and can even track usage by individual logon so you can see if someone is downloading videos that eat up a lot of your budgeted GB.
The Cradlepoint routers that provide Wifi As Wan (MBT95 and 1400) make monitoring data usage easy and they can be set to shut down when a certain percentage of usage is reached.
With the Cradlepoint router we use we get 99% usage of the available 40GB. No point in using only 85% as there is no rollover into the following month and the 15% not used is simply lost.
The Cradlepoint routers also enable you to track in real time so you can see if a spike occurred at 3 PM and then figure out what caused the increase in demand.
One thing we have done is avail ourselves of public libraries with internet access when we have needed to download very large files, as with operating system and software application updates. We have the updates set for manual and alert to we can control when and if they are done.
