The reason there isn't a formulaic, tight relationship between ISF (CF in the states) and I:C is because a plethora of additional hormones are involved in digestion, and consequent signaling to the glucose metabolism, that are not present during fasting.
Because of this, the insulin-sensitive tissues (chiefly hepatic, adipose, and skeletal muscle) react differently when glucose is present due to digestion, vs. during fasting (>3h after eating).
It "makes sense" when you think about it in terms of available energy during the evolution of more primitive creatures that we are evolved from. When food energy is available, your body want to suck it up like a sponge and store it. Hormones produced by the stomach and the brain when digesting combine with insulin to increase uptake by fat cells and the liver; the former converting the glucose in to fatty acids, the latter into glycogen complexes.
During fasting, BG has to be maintained from the body's stores. In the absence of those other hormones, insulin doesn't seem to work as well in part because any glucose stored away is simply canceled by glycogen being broken down to supply glucose to the body (and much more slowly, fat).
In the morning, with Dawn Phenomena, this vicious circle rages, and insulin can seem completely impotent.
For these reasons, eating something when your BG is high can actually help bring it down. The digestion hormones kick the storage tissues into high gear, and also suppress gluconeogenesis in the liver (as there is another source of glucose -- digestion).
This works for some, not so well for others. I'm one of the lucky folk that, if I have a small, balanced meal in the morning, my DP is totally controllable. If I skip breakfast, I fight it until lunch.
When your having a tough time correcting, eat a small snack (10-15g carb with a little protein and fat -- a few cheese and crackers is a good fit), then bolus for both your food, and the correction.
