The idea is to figure out the rate at which carbs are absorbed. How quickly all of the carbs you eat get completely absorbed.
The way to determine your carb absorption is to start out level and keep your basal running normally. Then eat a known amount of carbs and DO NOT dose for it. Watch your BG rise and eventually it should rise to the point where all the carbs have been absorbed. That length of time is the carb absorption rate.
You calculate this with your IC and correction factor. Where the carbs you ate and the dose matches where your BG gets to according to your correction factor. This is just used for the initial calculation, or any subsequent attempts to calculate the carb absorption rate.
Example, if you start at a BG of 100, and eat 40 grams with NO insulin. Suppose your IC is 1:10, and your correction factor is 40 mg/dL for each unit:
“I shoulda taken 4 units for that 40 grams.” (4 units x IC of 1:10). Oh no, now I am at 260. If I am at 260, my correction factor says I should take 4 units to get back to 100, because 4 units x 40 = 160, and 260 - 160 brings me back to 100…"
All the calculations aside, here is why the whole idea is a bit of pie-in-the-sky:
Carbs are absorbed by the villi of the small intestine, and there is a bit of a que for it to happen. If you eat a bunch, it will take longer for all of them to be absorbed. So larger meals are absorbed slower than small meals.
Also, some foods require less mechanical breakdown in the stomach, and will get to the small intestine quicker. Like fruit juice versus a solid food.
Activity affects it. And when you are active, food absorption will be slowed down, because your body will put that function on the back-burner.
The type of food affects it, such as they glycemic index, a cookie versus a potato.
Fat intake can affect carb absorption. And the fat content with meals might always be different, even if the carbs and food type are the same. A baked potato slathered with butter and sour cream versus the same exact amount of french fries.
So the idea is that if you see a bunch of IOB but you do not have to worry because you also see that you have food that is still going to be processed.
But again, it’s a bit of an idealistic scenario to think one number in there would always match all situations.