Sum numeric fields and multiply by a fixed number if a checkbox is selected.

You may want to implement an IF /ELSE condition with your checkbox.

You can use something like this:

if (this.getField("Checbox7").value =="Yes"){
event.value = (this.getField("Subtotal").value+ this.getField("Tax").value)*.035;
}

else if (this.getField("Checbox7).value =="Off"){
event.value = this.getField("Subtotal").value + this.getField("Tax").value;
}

You can use this script as the custom calculation script  of your Total field, for example.

Assuming SubTotal is calculated in a SubTotal field using a sum calculation applied to the SubTotal field that adds a number of number fields.

And assuming the tax is calculated as a percentage of the subtotal or a number entered into a Tax Field. I used a flat percentage based on location tax rates.

Try the following by creating a mouse up javascript assigned to the checkbox:

//------------------------------------------------------

var st = this.getField("SubTotal").value;
var tax = this.getField("Tax").value;
var total = this.getField("Total").value;

if (event.target.value == "Yes") { // This is the checkbox 
total = (st+tax)*1.035;
} else {
total = st+tax;
}
this.getField("Total").value = total;

//-------------------------------------------------------

I've tested this and it works for me.