Hi Lucie,
The thing to keep in mind regarding basal insulin doses is that they are designed to offset the production of glucose by the liver, not the glucose metabolised from the diet. On that basis, for example, if your basal was sufficient to offset the glucose generated from the protein in your roast dinner, what would have happened if you hadn't eaten it?
In my own experience, there ain't no such thing as a free lunch! In other words, even a carb free meal will need a bolus dose to cover it since almost everything we eat will at least partly be turned to glucose.
fergus