Any question answered...

I use my phone all day for work emails, personal email, twitter, facebook, instagram and forum all day.

I also often have to connect my laptop to the internet via the iPhones 3G to download documents when I am out on site. Which is heavy on data usage.

My average monthly usage is 336MB.

ah that's good news.
i definitely wont be using my phone for 3g plugged into my laptop! it seems that 250mb is the minimum most people offer, so i presumed it would equate to a fairly minimal amount of browsing/facebook/twitter etc..