HbA1c measures the amount of glucose that has attached to the haemoglobin molecules in your bloodstream and gives an indication of what your average glucose level has been over the past 2-3 months. Similarly I can get an indication from the length of my grass what the average rainfall has been, but it is not a measured average that you would get by actually collecting the rain. Using finger prick tests you could collect a lot of BG readings and work out a measured average of your glucose. The 2 calculators you mention give a conversion to and from HbA1c and a measured average. The problem with the measured average is that you would have to take a very large number of readings, including during the night, to make it accurate.