Custom column based on pivot table row totals

Is there any way to add custom columns based on the Row Totals result in pivot tables? Or otherwise customize that totals column?
E.g. I have a pivot table breaking down data by month, for last 12 months. Row Total shows past year total, which is good. But I would like to also add monthly average, i.e. Row Total/12.

Hi @Bungle
No, but couldn't you just breakout by the month, so it includes a subtotal for each month?
If not, then you would have to do it in SQL.

Thanks, not sure I understand though. I'm looking for a monthly average based on the row total, not a subtotal for each month.
Something like mockup below:

@Bungle Okay, that is not possible.