FireFieldChanged??

I have some code that adds some fields together.
I want to update the total everytime they tab out.

Function AddThemUp(){
var ADV_Registration_Bud = (nlapiGetFieldValue('custevent_adv_registration_bud')=="") ? 0 : parseFloat(nlapiGetFieldValue('custevent_adv_registration_bud'));

var ADV_Facility_Rental_Bud = (nlapiGetFieldValue('custevent_adv_facility_rental_bud')=="") ? 0 : parseFloat(nlapiGetFieldValue('custevent_adv_facility_rental_bud'));

var TotalOfFields;
TotalOfFields = (ADV_Registration_Bud + ADV_Facility_Rental_Bud);

nlapiSetFieldValue('custevent_TotalofFields, TotalOfFields);
}

Above is the basic Idea, I have put the function into both the Validate Record Function or Field Changed Function...I get a Stack OVerflow(LOL)

What is firefieldchanged and how do I use it...the manual doesn't give an example that I saw.