Echo “minimum payment due”

Hi.

I’m modifying the invoice template a little and would like to add in a field for “minumum payment due” (see example below):

Subtotal: $4,500.00
Discount: $200.00
Tax: $430.00
Payments To Date: $500.00
Total Payment Due: $4,230.00
Minimum Payment Due: $230.00

Now I can see the other fields in unified-invoice-page-active.php and I’ve added the field I need but how do I GET the amount?

I can see the TOTAL PAYMENT DUE is <?php echo wpi_get_amount_due
But what is the name of the wpi_ that I need to get for the minimum payment?

Many thanks.