You did not give any proof that the basis for the MIGO transaction is well maintained that it should not result in a number with 31 digits.
SAP has 64 OSS notes for J_1IEX_DEFAULT_EXCISE_DUTY
and they usually come only into the game if your conditions and PO pricing is really well.
So I think you have to do quite a bit more research on your own before anybody in sCN is able to help you.