Hi Luis

This is my method to sum 2 fields when checkbox is checked (formula) or to write something when is not checked (not formula) using Javascript (and only javascript, no formula !)

//When document is ready
$(document).ready(function() {
 
  //DO check_formula when the document is ready
  check_formula();
  
  //DO check_formula when the checkbox change
  $('#Field4-0').change(function()
	{
    	check_formula();
  	});
  
})

//Check_formula
function check_formula(){
  	
  	//Get Checkbox
  	var checkBox = document.getElementById("Field4-0");
          	
    // If the checkbox is checked, execute the formula
  	if (checkBox.checked == true){
    	
      	//GET Field 1 and Field 2
      	var field1 = $('#q1 input').val();
      	var field2 = $('#q2 input').val();
      
      	//Change STRING to INT
      	field1 = parseInt(field1);
        field2 = parseInt(field2);
          
        //SUM Field1 and Field2  
      	var total  = field1 + field2;
      
      	//SET Field Total
      	$('#q3 input').val(total);
      
  	} else {
    	
      	//SET Field Total = "No formula"
      	$('#q3 input').val("No Formula");
      
  	}
}

You need to know how to get ID or Class Name for each element.