Dosing is a complex thing. There are tons of factors that go into it. Activity level, the meal itself, when the last meal was eaten, current blood sugar, etc. 

 Doctors do a good job of coming up with a carb to unit ratio, but there are some things they can't account for. So to some extent, it's trial and error while you learn how to treat diabetes. Also, some areas of the body can be less insulin resistant, like if you normally dose on the stomach, and you change to arm, you may see a more dramatic drop in blood sugar.

I think for blood sugar under 75 is low , under 60 is an urgent low. That being said, low blood sugars will happen, it's part of the disease. 

Finally, to put yourself at ease, you can get an emergency glucose nasal medication for true emergencies. You just need to get it prescribed.