SUM Calculation of a Measure

[msc123]

I am using a measure to calculate the "count" of people that have achieved over 50% completion rate: 

.Count of 1 for people >50% = if([.Completion Percentage by Person]>.5,1,0)

I need to be able to summarize the "count" - and divide by the target, based a location that is set-up as a slicer.  I cannot find anything that allows me to summarize a measure, vs. a column.  I am very new to PBI and writing measures so any support would be appreciated. 

[AlexisOlson]

One way to do this would be to sum over each distinct person. It might look something like this:

SUMX (
    DISTINCT ( Table1[Person] ),
    IF ( [.Completion Percentage by Person] > .5, 1, 0 )
)