How to change calculated matrix into dimension when the Looker Studio set default aggregation type!

Hello, 

In my Firebase app property, there's a custom matrix named "goal_completion" which holds the values from 0 to 150 (in b/w any). I have created calculated fields to break the goal_completion value into respective categories i.e. 0 to 25, 25 to 50, 50 to 100 and 100+... 
I want this calculated field to be set as a dimension so that I can create a pivot table with dimensions like date, column dimension as the range (above) and users as a matrix.

I've tried cast(range function...) but did not work! Also, there is no option for changing the data type and aggregation (set to text, and auto by default).

What's the possible solution... 

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