Is it possible to edit a calculated field?

Question

Is there a way to make a calculated field editable, overriding the calculation? My form table calculates HST on pay amounts. But not everyone charges HST so I'd like the user to be able to put $0 in the field manually. I can create an HST option box for each row but...that's a lot of "contriving" and not a practical solution ( I don't think). Is there an easier way to do this?
Thanks.

Answer 1

The HST is variable because the form is used in all provinces.
In order to make it a no-brainer for the user, I have incorporated a check box on each row. If the check box is checked (value=1) then the HST calculation happens, otherwise the value=0. So this makes sense but doesn't work. I have adjusted the calculation order, doesn't make a difference.
Note: I've never used java but I used to work with Access databases so I do have an understanding of the language/logic, hopefully enough to help me learn this quickly.
Here is what I tried. HST option is the value of the checked Check box, which is "1". This code is returning the else value fine but otherwise isn't working and doesn't change when I toggle the Check box. Can you help me adjust this.
var h = this.getField("HSToption");
var f = this.getField("TOTALbeforeHST");
var g = this.getField("HSTPercentageValue");
if(h == 1){
event.value = f.value * g.value;
}else{
event.value = "0";