Working with custom field?

Hi

I'me developing wf with the next scenario:

1. I've created calculated custom field Delta = Deadline - Baseline Finish

2. If  Delta < 2 days, PMO must receive e-mail notification about possibility of breaking dealine

As i know, i should develop wf with the events. But what events should i use for with  scenario? LookupTable updated? Or Project Updated with the links to a task with deadline?

first activity "retrieve event data" or  "query project server"? How to find in "query project server" created custom field "Delta"?