Start with the cost basis of each asset along with a fair market value (FMV) of each asset both before and after the disaster. Be prepared to substantiate. The next piece of data needed is the amount of reimbursement you will receive from your insurer. What you are looking to calculate first is the loss in fair market value as a result of the disaster. From this loss in FMV subtract any reimbursements received from your insurer to arrive at the deductible disaster loss. Generally your deductible loss is the amount left un-reimbursed by the insurer.
For a basic example say you have a barn with a cost basis of $500,000 destroyed by a wild fire. You receive $150,000 from your insurance company for the loss. The fair market value before the disaster was $300,000 and after the disaster it is worthless $0 making the loss in FMV $300,000. The difference between the cost basis of $500,000 and the loss in FMV of $300,000 is $200,000. Subtract from the $200,000 the reimbursement you received from the insurance company of $150,000 and your deductible loss is $50,000. On the other hand in this example if you happen to receive a reimbursement from your insurance company of more than $200,000 you would not have a disaster loss to report.
For more information, review IRS Publication 584-B, Business Disaster Loss Worksheets. The listing of business assets is worthy of review, Others have found them to be particularly useful for helping victims list their business assets before the disaster as well as working through the information on business assets lost or damaged by disaster. I suggest preparing the worksheets in IRS Publication 584-B BEFORE preparing IRS Form 4684.