What event to use

I have a field that calculates a total. I'd like for the total if it's less than or equal to 250.00 to make a subform visible. The end user does not put their cursor in or leave the total field. What event should I use for my script?

Here's the script:

if (this.rawValue <= 250.00) {

    page1.superVisor.presence = "visible";

}

else {

    page1.superVisor.presence = "hidden";

}

Thanks,

MDawn