Historically toe is measured at the contact patch. And that makes sense, because it's a tire wearing angle. And the tires wear at the contact patch don't they?
The other thing is, it is a difference. So as long as you measure at the same place on each side, the camber angle is taken out of it completely (if they're the same side to side).
Realistically, the camber should be very similar from one side to the other. As in within a degree. So, even if there was a slight difference in camber from one side to the other, the effect on the toe would be VERY minor.
