DMEE for Italy. Mapping field or calculation method for the net payment with withholding tax

We have created a new three for the DMEE file for Italy. In the REGUP table there is the gross amount and the withholding tax amount, but the net payment does not seem to be stored anyway. Hence it seems we do not have a field to read for the mapping.

Table REGUPW is not field, but anyway I cannot find in the any field which I might use. The only way seems to create an exit providing this calculation. Is it correct?