Because arrears and interest are calculated differently from one jurisdiction to another, there is no one formula that will cover every support decree. You can get (or derive) the amount from your most recent support receipt.
Nearly all support payment notices include an accounting of the state of the current balance; the 'official' arrears figure should be listed as well.
Differences between the amount shown in the Arrears box and the amount on the support receipt are often due to interest, penalties and administrative fees paid on an amount in arrears. Because there is no set formula for determining these costs, OPTIMAL can not attempt to calculate this for you.
If the amount in the Arrears box needs to be adjusted, type over the existing number in the Arrears box before you save the record.