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

This example will also help determin the estimated uncollectible accounts receivables. The link is about the Percent of Sales Method to Calculate Debt Expense