I don't believe they use this method to calculate the value of subsequent segments.

I had a LAS-SFO-SAN connection yesterday and, in light of the awful SF weather, wanted to cancel the SFO-SAN segment because the delayed flights rendered the purpose of going to SAN moot.

The agent was unable to get the second leg refunded easily, because it kept saying that the value of it was $0. She was unable to determine the value of the second leg. Indeed, it was much cheaper ($262 total) to fly F from LAS-SFO-SAN than it would have been to book the two segments separately (they would have been $230-$257 each leg). Fwiw, these were all the cheapest F without any promo codes. I ended up getting the flight re routed to LAX because it was a suitable option.