One work-around that I'd be perfectly happy with, what if when you create a custom calculation, there's an option to also define how the total calculation should be calculated? Thus, the user could manually define how the aggregation works. Would that alleviate the complexity of assuming the aggregation type?
My thought is, e.g., a checkbox once a custom calculation has been added to "Define Total Calculation." When checked, that would open up another text box for the total calculation. The user could then define the Custom Column Total as a function of the other column total values or a function of the custom column itself, as appropriate.