Try using a layout script trigger of "OnRecordCommit". OnObjectModify only works when you modify the contents of a field manually, which will not work at all on a calculation field.
This one works whenever the current record is saved. so it should work for you.
The onModify script trigger only triggers when a user enters a field and
makes a change.
You may be able to use an autoenter calculation that includes the field you
want to do this.
Sent from my mobile device... Please excuse typos.
Both triggers do not work because the field SUBTOTAL isn't active.
Which is your calculation for the field: SALES TAX ?
It line items can be added in a different layout, you'll need to add triggering there too.