Custom Filed Module

Can i use Custom filed in facture.php file bcz using custom filed i apply discount on total amount in invoice pdf. same thimg i want to do on billed amount and Amount (inc. tax) filed

i’m really new here but i think it’s not a good idea to do things like total amount in customer field, because every modifikation of the sum in the invoice has to be written back to the main module to get no inconsistence