EXCLUDE certain amounts in calculation

I have the data below and am calculating efficiency rates for technicians performing various services onsite in customers' homes. 

The efficiency is calculated as:  EXPECTED Duration / ACTUAL Duration

Expected duration is provided for each item that is part of a work order ID.  Actual onsite duration is calculated based on geofence data (arrive time and depart time) using the address of the work order.  Since a single work order can have multiple items, if I sum actual onsite duration, I am overstating the amount.  In the example below (and in sample pbix), there are 3 items on the first work order.  The arrival time for the work order is 8:00 AM and depart time is 11:30 AM; thus, 210 minutes should be included only once in the efficiency calculation.

I can't come up with a calculation that will only count the ACTUAL Onsite Duration once for each work order.  I thought of adding a column that contained the count of work order IDs (for each work order), and then dividing the ACTUAL duration by that amount so that when summed, the amount would be correct.  Unfortunately, I couldn't figure out how to do that, either. 😀

I will be calculating efficiency by tech and eventually by Item no to find out which items the techs are struggling with (installing bunk beds or hanging a 60-inch TV, for example).

Any help would be appreciated!