Place this script as a "calculation" script into the 3rd field:
if (this.getField("FirstField").value == 1 && this.getField("SecondField").value == 800) {
event.target.value = 123456;} // where 123456 is the "fixed number"
else if (this.getField("FirstField").value == 2 && this.getField("SecondField").value == 900) {
event.target.value = 987654321;} // where 987654321 is the "fixed number"
else if (this.getField("FirstField").value == 3 && this.getField("SecondField").value == 1000) {
event.target.value = 657483920;} // where 657483920 is the "fixed number"
// etc.
else {
event.target.value = "";} // nothing