Determine the total estimated uncollectibles.

To determine estimated uncollectible accounts receivables: Analyze prior 5years of chargeoffs, determine the percentage against all trade receivables per year
Take average for 5 year period to be applied to current year.Apply percentage to current receivables for a financial reserve.Only actual writeoffs can be expensed as an offset

Here is an example of Percent of Accounts Receivable Method for Estimating Bad Debts Expense (http://www.drtaccounting.com/2008/03/percent-of-accounts-receivable-method.html)

This example will also help determin the estimated uncollectible accounts receivables. The link is about the Percent of Sales Method to Calculate Debt Expense (http://www.drtaccounting.com/2008/03/percent-of-sales-method-for-estimating.html)