how to get value from a field to influence another field

Hi All

I'm trying to make 2 fields one is "Grade" and the other is "Result". I'm trying to make it so that if I put in a certain grade, it'll show pass/fail. I googled how to do this but couldn't get it to work. Any pointers is greatly appreciated.

I have a validation script on "Result" as:

var fieldA = this.getField("Grade").value;
if (fieldA=="A" || fieldA=="B" || fieldA=="C")
event.value = ("Pass");
else
{
event.value = ("Fail");
}