Custom Calculation Script to apply a conditional discount

If you really want do this with just one Total field, use this:

event.value = Number(this.getField("Text1").value) + Number(this.getField("Text2").value) +
    Number(this.getField("Text3").value) + Number(this.getField("Text4").value); // keep on going until you have all your input fields
if (event.value >  398.40) {
    event.value = (event.value * 0.95).toFixed(2);
}

I probably would not just change the total without indicating why the total is no longer a multiple of $16.60 - I would have a filed called "SubTotal" (which is the total before the discount), and then another field in which I would calculate the discount, and then the new "Total" field that is the SubTotal - Discount.

In the Discount field, you can use something like this as your calculation script:

var subTotal = this.getField("SubTotal").value;
event.value = 0; // the default for this field
if (subTotal > 398.40) {
    event.value = (subTotal * 0.05).toFixed(2);
}

The script for "Total" would then be this:

event.value = this.getField("SubTotal").value - this.getField("Discount").value;