Making the total a fixed amount is not possible as standard with Magento. You can either give a fixed discount or a percentage discount. Both would only work if the initial basket total was always going to be the same. For example, if all products were $798 and they were only ever going to buy one, you would to a percentage discount of 50% in order to achieve the $399 goal.
I may be able to help if you could tell me more about how much and how many of the products would be purchased per order. It may be that you will need a custom extension to achieve this.