an HbA1c of 8.3% is an estimated average glucose level of 10.6mmol/l
It is estimated as we all glycate differently and so is about correct for most people.
In the UK they decided not to give this figure to patients because there are concerns that in some cases (for example anaemia) it isn't accurate.
It was on the calculator here it is 12.1 and just as i thought, nearly every website has a different figure. I suppose its like using another blood moniter.
The formulas used are quite different. I would suggest that the one from the ADA is more recent.
You don't need to bother about whole blood measurements as all meters in the UK now give results as if they were tested on plasma (until about a year ago some did results as in whole blood others converted to plasma; which is what labs use)