Ratio on DirectQuery model.

 
Hello everyone.
I would like to ask you for help in solving this problem that I have, since I am using directquery and some solutions that I found in the forum do not work.

I have values in a column that represent time (in minutes) that takes a process to complete.
I need to get the ratio of cases that have exceeded 15 minutes. so some how I need to first filter the cases that exceed 15min and then compare that number to the total cases.

the similar solutions I found here, when I tried to applied them, I get an Error msg that said that those DAX formulas are not allowedon DirectQuery models.

Hope I have been clear enought.
I will much appreciate your help on this.