Create a Script Object. In that object, you can have a function like this:
function addMyFields() // add values in the fields
{
var number1 = field1.rawValue;
var number2 = field2.rawValue; // and so on
var sumOp = number1 + number2; // + number3 + .... and so on
totalField.rawValue = sumOp; // display sum in total field
}
Paste the following code in the exit event of each field:
ScriptObjectName.addMyFields();
Let me know if it works for you.