Question: What’s the method to override the aggregation of a parent UD member when it is desired to have the UD parent be a calculation and not an aggregation.
Answer
It is not possible to override the aggregation and store a different value in a parent because UD parents are not stored members. The only way to accomplish this is make the parent member a dynamic calculated member and define the calculation you need on that member.
Source: Office Hours 2019-11-26 Partner Enablement