A sum insured is a rebuild cost and a ledger is a purchase cost
Sum insured on the right basis is the single largest decision on a commercial policy, and it is usually copied forward from last year. Reinstatement value moves with steel, cement, machinery prices and the rupee. Book value does not, and the average clause does not care which one you used.
The question that settles itIf the whole shed burned tonight, would this figure rebuild it at today's prices?