Originally Posted by
fizz57
Perhaps that's when the Mach cone hits the wingtip?
This does not really explain whys the drag lines for 0°, 30° and 45° wing sweep all meet at M2,1.
The cone angle of the Mach cone is Phi=arcsin(1/Ma).
That gives a cone angle of 28,4°, equalling a sweep of 61,6°.
Edit2: Deleted after valid comment from @Hazelnuts.