Couldn't really tell you about absolute measure of blood sugar - I mostly just watch to see if I start to feel a cold sweat washing over me, say after I've done a heavy workout + carido or a long time since I last ate. That's just your body tellng you all your reserves have been burned up & time to eat something.
Not sure what direction you are looking to take this info, but re: low blood sugar (e.g .for diabetics) is 80.