heheh, that's a good formula!
Basically and A1c is an average of what your blood sugars have been during the last 3 months or so. But, like all averages, it is always wrong. My last one was a 6.7, which the ADA considers good control (the AACE wants 6.5%). So I'm pretty well controlled right? wrong... during that time period I was having extreme highs 2hrs after meals and extreme lows at 4hrs. That is not well controlled. Those highs averaged with those lows gave me a number right in the middle!
IMO, It should be used as VERY rough guideline for how you have been doing. I really just look at my daily patterns, that what I care about mostly. Although, I do admit that I have a personal goal to see an A1c under 6. And that's with no bad lows.