Percentage deposit on invoice

I want to make invoices that have a field below subtotal (or anywhere near that) that shows a 25% deposit that needs to be paid. This should be an automatically calculated field (25% of subtotal); retainer invoices don't work because it does not allow to calculate a percentage of the invoice.

Is this possible?

Another option would be to be able to add the 25% amount in the email and not on the invoice. Key is that it should be calculated, no manual entry.