6 off in 40 = 3 in 20 = 9 in 60: initial error = 9º

                To recover 6 nm in remaining 60 nm to B

                Requires further 6º                                 

                9º + 6º = 15º                                                          9º

                                                     40nm                  60nm       6º

                                                                     6nm

                                          9º                                                                   6º