Anyone who has read a lot on numbers and statistics knows that using 'averages' (also known as mean) for anything is a bad solution. You could be having some ridiculously high, highs (from stuffing with high carb foods) being offset by some ridiculously low, lows (using fast acting insulin to counter the carbs) and still get an average that makes you feel you are ok! Admittedly, that is an extreme example, but you get the idea.
If one really wants to look at the trends or get a clear picture develop a medium, a list of readings staring with the highest working down to the lowest. Then find the center of that list of readings, which would be your medium. But, what would really tell the story, is to count how many highs over 140 you have had compared to how many healthy readings....
Just food for thought, using averages is dangerous without further evaluation.....