Assuming there is an association many Parts belonging to one PartsQty
You could do two things (depending on if the sum is retrieved or calculated more often): Make SumUseQty calculated and call a microflow that retrieves all parts associated to the PartsQty and use SUM (Aggragate List) to calculated the value.
Or, SumUseQty can remain a regular field and make sure that on save of any Part Object, the value is recalculated.
You can take a look at this part (and the next section) of the Rapid Developer Learning path to decide what is the best option for you.