Hi,
Create a "Statistical" Condition type in the procedure where you need to calculate VAT and introduce it for the calculation of "Subtotal" which will be the base amount for VAT calculation. At the time of PO and MIGO you dont know this value so your VAT is incorrect. Just before MIRO when you know the value edit the PO to introduce this "Adjustable amount statistical condition type". Now the VAT amount will be correct in PO and you can post the MIRO.
As per my knowledge you cant introduce a statistical amount for any purpose in MIRO.
Regards
Binoy