Sorry if I have got the wrong end of the stick, but the 1 in 60 rule is based on three straight sides of the triangle. Certainly enough to give you a ballpark figure which most of our flying calcs are based on. The ARC will always give you a bit more distance than a straight side. The circumference of a circle is 2 x Pi x radius. Pi being 3.14 and quite a few more decimal places. That is why the differnce is 2.8nm. 1 in 60 10 nm = 60nm. With circ calc = 2 x 3.14 x 10= 62.8nm. However as I said not a huge difference, on a 40 degree arc the difference will be less than 0.5 nm. Hope I am barking up the right tree!?!