Percentiles based on group summations

Hello,

I'm borrowing from a previous unanswered question found in the link below:

http://community.powerbi.com/t5/DAX-Commands-and-Tips/percentile-group-by-multiple-categories/td-p/77959

I have a similarly structured table that has data that looks like the one below. I'm looking for percentile that is dynamic (I'm thinking it's going to have to be a measure) based on a filter or set of filters (was thinking ALLSELECTED would have to be used but I'm unsure).

I need to see the 50th Percentile Lead time by quarter and/or by site dynamically in a visual. I did look into the percentile visual but could not get it to work the way i require it to work. The Materials are unique to a site, so that is not a concern. 

I would have to see 50th Percentile Lead time by site, 50th percentile lead time by quarter and then see 50th percentile lead time by quarter and site together.

If this data were to extend to 2016Q3 and 2016 Q4, would it be possible to treat the values compared in percentile as the sum of each quarter?

Please help!

Edit:

I believe I'm looking for CALCULATE(SUM(CALCULATE(PERCENTILE(TABLE[Lead Time]),.5), ALLSELECTED(Table[MATERIAL]))

but the sum function only accepts a column reference as an argument.