Just an update from me also:

At 252 miles I went down to only having one full bar plus the two little bars at the bottom left. So I have already got more than I would normally get, but still touch and go as whether or not I will get through the 300 mile barrier. I will probably reach the end of my challenge in the next day or so(I think I'm right in saying that the petrol light comes on when there's only the two little bars left? I'm having doubts in my mind now as to whether it might be when only one of them is left), so will let you all know before long.
Scott, with regards to yours, I obviously don't know how far down the gauge it goes on a Focus before the light comes on, but I'll guess 412.