Probably preaching to the choir with this post, but still:
I test twice with my meter every time I have a result >250. Sometimes the variations are huge, one example from last week: 428 mg/dl vs. 303 mg/dl. In this case, the deviation from the true value (which of course I don't know) has to be...
Please read over here, at my D-blog Pricked Finger in the Pie: Meter Accuracy