Im trying to get a check box in adobe to (When checked) to subtract

this is the code im trying to use, Im trying to get a check box in adobe to (When checked) to subtract "TTLCalculated" And use that value in the next part of my code, wanted to know if what im doing wrong, im a new to javascript, would really appreciate the help!

var DA = Number(this.getField("Downpayment*").valueAsString);

var DA2 = Number(this.getField("TTLCalculated").valueAsString);

event.value = (DA-DA2);

if (this.getField("TTLCalculated").value!="Off") event.value=(DA-DA2);

else if (this.getField("TTLCalculated").value!="Off") event.value=0;