Conditionally Formating Subtotal Output

I created a subtotal for my column group by right-clicking on the group and selecting subtotal. I would like to add the condition to the subtotal, that if the value is less than 0 to print zero and not the negative subtotal amount. Is there a way to do this? I haven't been able to find a way to add an expression to the greyed out subtotal field. Thanks.

I would get rid of the autogenerated subtotal, create another matrix that sums the field and conditionally edit the expression of the Sum matrix.

I have also found that the autogenerated subtotal feature has much to be desired.