Purchase line item net amount is not updating properly after customization

I am very new to Microsoft Dynamics AX.

I have added one customized combo box in the purchase edit line section. I have updated the price unit price value based on the selected valued of the combo box. So, I have modified the modifiedField method of the PurchLine table. It is working properly, but the problem is "Net amount" field is not updating (calculating) the amount properly.

Thanks in advance,

Hari