There is Percentile function that can help you in calculating upper and lower quartile beside median.
Create 3 calculated fields with following formulas
Above created calculated fields can calculate quartiles at any granularity.
I think this approach would work if I were calculating the quartiles based on the raw data. However, my data has already been pre-aggregated with the counts. Did you look at the attachment?
Thanks in advance for any additional insights that you can provide.