Think about HbA1c

Marina,
First let me say that you've made a dramatic improvement in your HbA1c in the relatively short time since your diagnosis. Congrat's to you on that.

HbA1c is measured by testing the amount of "glycation" of our red blood cells. In simple terms, that's how much glucose is sticking to them at the time of measurement. Since our red blood cells die off in "approximately" 3 months, it's a moving number with new glycated red blood cells replacing the older ones. In your case for example, the old cells were more glycated than the newer ones so your HbA1c showed more of the lesser glycated ones, hence a lower HbA1c reading.
All that said, there's no way from just looking at a HbA1c test result to know exactly how much of that number is coming from the last month and how much is residual from the previous weeks. Suffice to say that the test is "weighted" by the most recent weeks.

Then there's the fact that there are several different math formulas that describe the relationship of HbA1c and blood glucose levels. There is so far no standard way of making that calculation, so you just have to chose which formula to use and be consistent if you want to make comparisons over time.
One formula would say your 12.8% represents a bg of 324mg/dl and your 8.7% = 201mg/dl. Other formulas would give somewhat different numbers. What's important is not the actual numbers, but the fact that you've made significant progress in the right direction!

Since you're new and I don't know how much you know about the importance of what's to be learned from the HbA1c numbers, I'll just give you the main reason we care about that test result---
HbA1c (Glycosylated Hemoglobin A1c) is our best (only really) indicator we have for our risk of getting one or more of the horrendous complications that can befall diabetics. The basic rule is that lower HbA1c = less complication risk.
So what "should be" our goal for HbA1c? The best possible would be to have it be in the range of the normal non-diabetic which at most labs is ~4.5% to 6% or in some cases just stated as "under 6%).
The truly non-diabetic will have a HbA1c near the low-end of that range rather than near the 6% top of the range, so that is the goal that many of us here on the list aspire to. Some of us make it, some of us are still trying, but we still keep that goal in mind. There are some other lifestyle factors that may make a higher than ideal HbA1c acceptable, but that's a discussion for another time.

I'm curious why you're asking about your past month's bg number in relationship to your HbA1c when you should be able to get that information from your own bg testing and the log you've kept of those readings, or perhaps you have a tester that can calculate that average for you, which some of them do nowadays?

Sorry for the long-winded answer to your short question, but not knowing how much you already know I thought I'd give you more information that you asked for, albeit not the answer to your specific question which really has no answer!
Roger, T2, etc...

I have a question regarding A1C numbers. When diagnosed Oct 2, my A1C was
2.8. I had another A1C test done last Tuesday, Nov 17 and the result came
ack 8.7. This was mostly achieved through change in diet and mild
xercise. I realize this is a 3-month test so how would I be able to
alculate what my average numbers were the past month only? I'm
ath-challenged, as you can tell ;-)
hanks
arina