How to print tax summary on Purchase Order

Hi, I think I have a blind spot for something that should be simple.

I want to print the tax summary at the bottom of my purchase orders.
On a Sales Order or Invoice form I can customise the "Printing Fields" -> "Footer" to include Subtotal, Tax, Total.

However, when the form I'm trying to customise is a Purchase Order then I don't have the opportunity to tick these fields because they're missing.
I only see Message, Subtotal, Total and Barcode, nothing to do with the Tax breakdown.

Can anyone throw some light on this?

THanks,

Paul