Custom field added to standard data entity in D365FO

I am working on ledgerjournalEntity ,I added the new field in its extension and also in the staging table. this field supposed to be  a calculated field .
1. where to add code for the calculation (right now I have created the extension class and added code for createjournalHeaders()). But the breakpoint is not hitting .
2. how to debug the import process , I can debug all the other things but when I put breakpoint on the LedgerJournalEntity's copyCustomStagingToTarget() it does not hit ?
3. The data is visible on Staging table but when I check for target the data is not there