If I understand correctly, you can create a new clinic dimension with
if [Facility] = 'ROS' and [Clinic] = 'NSG' then 'OSS' else [Clinic] end
and build the group from there.
In the attached, I only grouped OSS, you will have to redo the rest of the groupings. (Sheet 4)
sample3(mc).twbx 250.3 KB