Why do Spitfires turn better than Hurricanes?!

You misread me, Tom. In REAL life turning circle, 10% difference is a big difference, here on this point I agree with you. What I was talking about is the error induced by numbers resulting from thump rule calculations. The friend of Mr. Bungay made these thump rule calculations for the turning circle for the Spit, the Hurri and the 109. Now from my experience thum rule calculations contain errors that can be huge with respect to reality and beyond 10%. So if a thump rule says the turning circle of the Spit is 690 ft and we assume the error in this calclation is 10% then the real life turning circle would be expected in between 621 and 751 ft just we cannot tell at which end the real life turning circle would be in fact.

Now if the same thump rule says the Hurri has a turning circle of 660 ft and we assume that also the error is 10% then the real turning circle of the Hurri can be expected to be in between 594ft and 726ft. Again we cannot tell at which end the real life turning circle of a Hurri would be in fact.

So it is absolutely conceivable just from judging the numbers and an assumed error of 10% that the real life Hurri could have had a larger turning circle than the Spit. Or vice versa.

Because the assumed error of 10% is larger than the calculated difference between both planes. Indeed from my engineering experience the error will rather be bigger than smaller. 10% error would be actually quite good. Even aerodynamic coefficients calculated with highly sophisticated numerical methods using finite element methods can be subjected to an error of that order of magnitude with respect to wind tunnel tests and 5% have to be considered as really good.

So basically the numbers as given by Mr. Bungay have to be taken very carefully.