How accurate is the usage data?

[#1]

Hi all,
I'm just wondering how accurate the usage data is, today for example I've done some emailing and a bit of surfing, mainly facebook but have somehow managed to use half a gig which has taken me over my usage allowance with 3 days left until the end of my billing period.
Tia
Jon

[#2]

Ignore me, I've just remembered that I went on google earth earlier... must stop that really.