You post the expense to Bad Debt Expense and credit Allowance for Doubtful Accounts. The percentage you use depends on whatever policy your accountant set. The percentage you use depends on whatever policy your accountant set. Something like:
Debit - Bad Debt Expense
Credit - Allowance for Doubtful Accounts

That keeps your AR aging intact and still shows the expected write-offs in your financials. When you eventually write off a specific invoice, you hit the allowance instead of the expense again.