Grand Total erratic

I have a simple form in which a quantity field is multiplied by a set amount to reach a subtotal (using the Simplified Field Notation: Field Name * Cost). There are 25 subtotal fields which are then added together using the "Value is the sum of" option under Calculate in Acrobat. That works fine until the user goes back and alters one of the quantity fields. The subtotal changes correctly but the grand total field does not.

Is there a javascript I can use to correct this or am I doing something else wrong in Acrobat?

The only actual javascript I'm using is in 6 subtotal fields which require a number to be subtracted before the subtotal can be reached:

var a = getField("IceSkating").value;
event.value = (a * 20) -20;
if (event.value < 0) event.value = 0