Avg Days Late on Customer Accounts Screen in Receivables in R12

Hello

We are in R12.1.3 version, I am writing a custom report on Customer Data, for that I want to know the formula used to calculate Avg Days Late, can anybody help me on this?

For example, I am seeing Avg Days Late on Customer Accounts Screen was 165

I have total 4 open invoices as below with Days Late as below

INV#1             411 Days Late

INV#2             163 Days Late

INV#3             130 Days Late

INV#4              124 Days Late

Thanks