Blood sugar / glucose monitor?

I asked the diabetes clinic how they come up with a 3 month average with 1 set of blood work? They said their general "formula" is your blood reading-6X2+6= 3 month average. So my level was 7 (Canada), so 7-6x2+6=8 is my 3 month average.
Actually they usually can figure back up to 90 days or there about.