Setting Focus on a Field after Validation

I am currently working on a form that requires the entry to be validated based on another fields total. I have two problems
1. For some reason I cannot get the script to work in the validation section, only in the onBlur for the fields custom script.
2. I want to set focus on the previous field and then zero out the entry. Here is the code I have currently: ("tper" is a running total of entries)
 
if (this.getField("tper").value > 100) {
// User alert
app.alert("Total percents entered must = 100%. Please check your entries.");
this.getfield("the field that had the focus").setfocus();
event.value = 0;
}
 
I believe I may be calling the previous field improperly. Any help is appreciated.
 
Aaron