need help on 'BeforeFormSubmit' event

[Chakkra]

I want to achieve this simple calculation (below) in my details page. This is where the user enters new lab_fee and the update should take place for the value of TotalFee. 
It should automaticcally display the value before i hit the Update button on the details page.  
....
var v1 = document.getElementById("EditRecordlab_fee").value;
var v2 = document.getElementById("EditRecordc_discount").value;
var v3 = document.getElementById("EditRecordcredit_charge").value;
var v4 =  v2 + v3 - v1;
document.getElementById("EditRecordTotalFee").value = Round(v4,2);
....
According to Caspio Bridge 13.0, it was suggested that the below javascript could do the job. i guess for my case i have to put some codes in the //do somethig. Or I might be wrong. 
Can anyone kindly help me on this? 

<script type="text/javascript">
document.addEventListener('BeforeFormSubmit', function (event) {
// do something
});
</script>  

 

[MayMusic]

You can try a "calculated value" form element instead of JS.  By default, the form element is text, change it to  Calculated Value and select the fields you want to use in your calculation:

[@field:discount] + [@field:charge] - [@field:lab_fee]