Per my knowledge, it is not possible to show the average of detailed values in subtotal row and show average of each subtotal in grandtotal row. By default, both grandtotal and subtotal calculate results with SUM aggregation. Even if we generate a measure to get AVERAGE rather than SUM, the measure will calculate based on detailed rows. That is to say, it is not possible to calculate based on subtotal level.
Community Support Team _ Yuliana Gu If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.