Hi Sunny,
For this you have to write a logic i.e.
Take the invoice number as test data (say 9000000000) and pass this to VBRK-VBELN and pick VBRK-KNUMV. Now pass this KNUMV to KONV-KNUMV and total up the value of KONV-KWERT against KONV-KPOSN and multiply the total of KONV-KWERT with KONV-KKURS (Exchange Rate) where KONV-KSCHL = "PR00" (Here take KONV-KPOSN = "10".
Through this you can surely get Assesable Value.
Regards
Abhishek