As the insurer established a plausible theory of harm outside the claim, the insured was required to establish some reasonable basis of apportioning the damage; absent this proof, the insurer has no liability under state law.
http://www.ca5.uscourts.gov/opinions/pub/17/17-10914-CV0.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.