Hello Whiskysmum
We started with the advice in Gary Scheiner's book Think Like a Pancreas, and then fine tuned it after experimenting.
The book says, divide 1,500 and 1,800 by your average total daily insulin units (basal and bolus). The answers give you an approximate range of how much 1 unit is likely to lower your blood sugar. But because this is an American book, you then need to divide the answer by 18. So, suppose you take an average of 30 units a day. The range in US money is 50-60, which works out at 2.8-3.3. I think it works OK, but the numbers will always need tweaking depending on your typical diet.
Hope that helps. The book is very good on lots of other topics too, and it does include UK numbers in brackets pretty much throughout.
Bye for now
Catherine