Percentage of customers from a district

I need to determine how best to model the percentage of schools in a school district that are customers of our product. I have a measure for distinct count of schools that I can view by district, but I'm not sure how best to compare this to the total number of schools in a district. I need to calculate, among other things [total schools in district] - [ customer schools in district] and [customer schools in district] / [total schools in district]. What is the best way to model the total number of schools in the district? Would this be a measure in a fact table or an attribute of a dimension? Also, I'm using the Microsoft BI stack and SSAS.

I would treat it as an attribute of a district.

if granularity of fact is district and you want to calculate all these calculations at report layer then store this in the fact itself, else as an attribute in dimension.