Ohhh I see, that is tricky! In this case, the next best step is to run a trace. Do you know how to do that? Trace Helper Utility for Epicor ERP 10 - Epicor ERP 10 - Epicor User Help Forum (epiusers.help)
When you run the trace, it will keep a log of all the methods that are triggered. Then you can use that method to trigger a BPM. I am still not sure about pulling the data from the calculated fields with a BPM. I hope someone else with a bit more know-how can chime in here.