On a mercator:

 

Scale    = Chart distance    =   Scale at equator x     1   .          

                 Earth distance                                            cos lat

 

Transpose:       Chart distance = Scale at equator x Earth distance      

                                                                     cos lat

 

Since it is the same chart distance at 45ºN and 30ºN:

 

 Scale at equator x Earth dist. at 30ºN = Scale at equator x Earth dist at 45ºN                                     

                            cos 30º                                                 cos 45º

 

Or:                   ED at 30º  = ED at 45º

                        cos 30º         cos 45º

 

ED at 30º = ED at 45º x cos 30º  =  70 x 0.866 = 85.75 nm

                              cos 45º                          0.707