Determine field value based on Radio buttons - custom calculation not working

I'm trying to do something that should be simple, but it's not working.

FIRST

I think I have the right code, but not sure:

var frequency = this.getField("1_Year").value;

if (frequency == "Off") {
Amount.value = "100";
}
else if (frequency == "On") {
Amount.value = "200";
}

Would this output "100" into field "Amount" for off and "200" for on?

SEDCONDLY:

when I type this into a custom calculation field and hit ok, acrobat closes the window as if it's accepted it, but when I look at the box, it has gone back to "value is not calculated", which is obviously not what I want.

Any help?