It's Like Landing on an Aircraft Carrier...
The "target" BG range is so narrow, compared to falling short (and crashing right into the aircraft carrier, sinking the ship in the process) or overshooting the strip (and falling somewhere in the sea of high blood sugars that lay beyond the carrier).
This is a typical graph for blood sugar readings. That tiny green strip is considered the "target" range. That narrow strip goes from approximately 80 - 180. You can see, just looking at the picture, it's a tight area!
The margin for error is very slim, with the penalty of a low blood sugars below the "safe zone" and high blood sugars above. With that in mind, let's talk a bit about the calculations that take place when dosing insulin for correcting high BG's or calculating for grams of carbohydrates.
My insulin to carb ratio is 1 unit for every 7.5 grams of carbohydrates. For a correction factor, 1 unit will drop my BG by 35 points. So, I'm assuming based on those two figures that 7.5 grams will raise my BG by 35 points, or 1 gram by about 4.7 points. All of this is altered slightly by any number of things - exercise, being sick, stress, etc. But for the most part, these are the numbers used for the calculations.
It doesn't take much of a miscalculation to send me out of range, one way or another. If I miscount my grams of carbohydrate by 21 grams (slightly more than 1 piece of bread), I will be off target by 100 points! It is deceptively easy to goof up by a slice of bread one way or another, especially when dealing with food in large quantities (one of my other problems...). And how easy is it to over treat a low by 21 grams or better?!
Now, this is an extreme example, being off 100 points, but you can see how being off by even half that amount (half a slice of bread) means 50 BG points off target one way or the other.
An aircraft carrier is huge, until you see it relative to the ocean it's floating in...
The best we can do is take a shot at it, and hope that if we miss we'll still be close enough to swim home!