If cell A1 says: #N/A N/A then populate from A2. However if A2 also says: #N/A N/A then populate from cell A3. If A3 is also saying: #N/A N/A then populate from A4.
I'm able to do this for just one switch, so I can get it to populate from A2 if A1 says it. But I'm not sure how to do it for all...