Text Field not calculating properly

I'm hoping someone can help me figure out how to resolve my text field not calculating properly.

I have a text field name 'TotalOTHrsEmp1'.  I click on 'Calculate' and choose 'Value is the' and pick the fields I want to calculate the hours.  There is one particular text field I am having a problem with calculating.  It's a checkbox that populates '1' to another text field.  When I check the box the hour doesn't add up, but when I uncheck the box the hour does add up.  That's not what I want, I want the '1' hour to add up in the 'TotalOTHrsEmp1' text field when the box is ticked.  Here is what I have set up...

The checkbox is named 'TravelTimePdEmp1'.  The text field is named 'TTPdEmp1' and in this text field I have a custom calculation script as follows, 

var a ="";

if (this.getField("TravelTimePdEmp1").value == "Yes"){

a="1";

}

event.value = a;

Thank you in advance for any help with this!