Steps to be followed.
1.Open PO in EKPO And get net amount and get tax code MWSKZ .
2. Execute function module CALCULATE_TAX_FROM_NET_AMOUNT.
3.Pass Company code , Tax code , currency and amount in I_BUKRS / I_MWSKZ / I_WAERS / I_WRBTR respectively.
4. You will get the data in T_MWDAT.
Regards
Pankaj Jindal