When one has high blood sugars, the doctor says take 1unit for each 5.0 it is high. But does that start from, 0.0 or from your normal range. So for example if it is 20.0 mmol/L would you have 4 units of insulin or 3 unit. Does the 3 units bring it down to 5 mmol/L. Would the 4 units do, or bring you down dangerously low?
From the normal range. otherwise you would go low.
To correct high blood glucose you need to settle on an actual target. My target is 4.7 mmol/l (85 mg/dl). Doctors generally like to aim higher, like 5.6 or 6.7. A target of 0.0 would drive you too low. You need to pick an actual glucose level within the normal range.
If your blood glucose measures 20.0, then take 20.0 and subtract your target blood glucose level, let's say 5.6. The difference is 14.4. Using your doctor's recommendation to take 1 unit of insulin for every 5.0 that it is high, you would divide 14.4 by 5.0. 14.4/5.0 = 2.88. That means you would take 2.88 units of insulin to correct a 20.0 glucose level down to 5.6.
In summary here's your formula: (High blood glucose - target blood glucose)/5.0.
The 5.0 number that the doctor gave you is also known as your "insulin sensitivity factor" or ISF. That's the amount of blood glucose drop that will occur when you take one unit of insulin. This is not a fixed number that will always remain the same. Your ISF will change and may even be different for different times of the day. It will also change when you get sick, sometimes by a lot. I would assume that your doctor's number of 5.0 is conservative and you may need to change it to a more aggresive number like 4.0, but its a good place to start.