We'd like to be able to set Second Payment - Due (under Properties - Rules) to 0 (zero)

We'd like to be able to set Second Payment - Due (under Properties - Rules) to 0 (zero) because we want the guests to make the second payment on their arrival. Currently, we have to set this value to 1 and say in terms and conditions that the second payment is due on arrival but this creates a lot of confusion.