It is a great book but the graph that you refer to has to be taken with care. As Bungay explains a friend of him who is working as an aeronautical engineer has calculated them - in a simplified manner as far as I understood. Being an aeronautical engineer myself my strong guess goes to that the friend of Mr. Bungay used some simplistic considerations, derived from these some very simplified equations and then put in numbers leading to the results depicted in the diagramm. For some basic estimations this approach is quite all right and acceptable but a good engineer should always (ALWAYS) be aware that this approach is far from reality. It may give some hints in terms of relative behaviour but honestly from my experience a difference of 10% between two airplanes will not say that one plane turns 10% tighter with this kind of approach. In fact they have to be considered practically equal or better a 10% with simplistic calculation is not significantly enough to make a statement. Now if the difference would have been 50% then one could with some confidence say that one plane turns better than the others although one still cannot say by how much one plane turns better. There are far too much uncertainties in this kind of engineering approach to believe in their results like some do in the words of The Book.