Minimum payment amount displayed on invoice

[verygrap]

Hello,

 

I hope you can help. I need to display on the client"s invoice a "minimum payment amount". This field is for display purposes only and is a simple calculation of 25% of the invoice total. It is not required to be written to the database.

 

How can I add this to my invoicepdf.tpl so that the calculation is done on the fly.

 

Thanks in advance for any help you can give me.

 

Cheers,

Sharon.