For POST_AGG table calculations, the partition(s) used in the calculation needs to be in the visual.
PRE_AGG should work in this case -
sum(maxOver(Budget, [ID, Category], PRE_AGG)/countOver(Budget, [ID, Category], PRE_AGG))
For POST_AGG table calculations, the partition(s) used in the calculation needs to be in the visual.
PRE_AGG should work in this case -
sum(maxOver(Budget, [ID, Category], PRE_AGG)/countOver(Budget, [ID, Category], PRE_AGG))