Index Table in FIELD_EXIT

I've created a field in the output VAT in transactions ME21N to suggest the code in line with historical data. To find need the VAT and the product supplier. With the function DYNP_VALUES_READ recovered the data, but I'm not the STEPL (index) of the current record. I can not find the value of the index. Does anyone know how to recover this index within the FIELD_EXIT, or another way to solve this problem?