How field value "per_billed" is calculated ,backtrack debugging

In purchase receipt ,on more information section there is field name % Amount Billed (per_billed).
we want to create a custom field calculating “quantity that billed” (qty_billed),this field should be working same as “per_billed” but instead of amount it should calculate “qty”.

I am not getting how this field “per_billed” is calculated , I have seen purchase_receipt.js , and and searched for variable name per_billed but not able to understand how this field calculate values .

I like to use inbuilt functionality rather than creating new one .

Is there anyway we can backtrack this field functionality??
does any IDE is helpful in this ??

Please suggest