You can use something like this:
var v; if (this.getField("r51").isBoxChecked(0, true)) { v = (this.getField("r18").value/1000) * 13; } else { v = this.getField("r18").value*.4; } if (this.getField("r20").isBoxChecked(0, false)) { v = v/2; } event.value = v;