This is something that should be addressed in contractual language.

For instance, I think it would be reasonable to define "deviation" as changing your itinerary so as to arrive at the city of first departure on a flight different from scheduled.

I've had multiple-day/multiple-leg deadheads where I booked a different flight the first day but caught up with my scheduled deadhead on the second day. On one occasion, my scheduled deadhead on the second day cancelled. Even though I technically deviated, skeds took care of me as if I had not deviated. It was the reasonable-man theory in action, though I was very uncomfortable being so exposed. Ever since, I've been very reluctant to deviate on the front end of a trip.

As I said, I think the best answer is to put some reasonable language in the next contract.