Well, that's a question more of engineering than of chemistry. Technically, as the concavity of the sigmoid plot changes, the first one is done. But a few drops may still be carrying over in the rig. One could say, at the midpoint of the sigma plot, all has come through, but you might not see the concavity or enough of plot to notice. If you were watching the receiving vessel, you might see a refractive index mixing, as the first drop of water hits the collected methanol, then again, you might not. Kinda hard to be very precise, with distillation, sometimes.