Create a dynamic Leadtime filter, calculated only on the IDs which contains both selected values.

Hi PhilipL 

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How it works ....
Create "inactive" relationships

Create measures to get start and end time using the relationships

Start time = 
-- get time using start relationship
CALCULATE(
SELECTEDVALUE('Datatable'[Timestamp]),
USERELATIONSHIP(StartActivities[Start Activity],'Datatable'[Activity]))

End time = 
-- get time using end relationship
CALCULATE(
SELECTEDVALUE('Datatable'[Timestamp]),
USERELATIONSHIP(EndActivities[End Activity],'Datatable'[Activity]))

 Create measure to calculate the lead time

LeadTime = 
-- get lead time unless the start or end is blank or the end is before the start
SWITCH(TRUE(),
  ISBLANK([Start time]) || ISBLANK([End time]), BLANK(),
  [End time] <  [Start time], BLANK(),
  [End time] - [Start time])

 Create a measure to get the average

Average LeadTime = 
-- get average as decical value, ignoring blank values
VAR decimal_average =
AVERAGEX(
    VALUES('Datatable'[Case]),
    [LeadTime]
    )

RETURN
-- convert deccimal to hours and minutes
FORMAT(decimal_average, "HH:MM")

Then build your report 

Thanks for the clear description of the problem with example data. I wish everyone did that!

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