Calculation in PDF Form

There are 3 options for calculating field values. The first two, selecting field and simplified field notation have no provision for controlling the lines executed by JavaScript. This is only available using the Custom JavaScript option. This will require identifying each field's object and then applying logic to tell if all six items have been selected. You will have to check if a field is checked or not by testing the value of the field. A value of "Off" indicates no selection has been made. One could test each check box and count or sum as needed or place the value of each field into an element in an array and then filtering out the "Off" elements. One can then sum the elements of the array and if the number of elements in the array is 6 then apply the discount.

You can use this code as your custom calculation script for the total field:

var total = 0;
for (var i=1; i<=6; i++)
    if (this.getField("Box "+i).value!="Off") total+=50;
if (total==300) total-=50;
event.value = total;