You would need to compare your meters 30 day adverage with several HbA1c's to get some indiction of how far your meter is off, and even then you can only use it as a rough guide to what to expect. As the meter only works out the adverage from test carried out, so has't the ability to work out what happened between those tests.
I keep a eye on my 30 day adverage, as this can be a indicator that overall my levels are creeping up on a daily bases which may not be apparent. I check my 30 day adverage once a week, to see if it is going down, steady or creeping up...
If it steady then I'm not worried.
If it goes up then I look very closing to the 'why'
If it goes down then I'm chuffed but still make sure that this isn't due to hypos altering the results...
On my present meter, I've worked out that I'm looking at my HbA1c being around 1.6mmol/l above what my HbA1c,s comes in, but other meter I've had have had different differances ranging from about 1mmo/l to 2.5 mmol/l, I also checked the 30 dayg adverage, when I have my bloods taken,