Im not an Excel expert, but to me it seems that you are treating words as if they were numbers in the final bit of your code. The final bit of code I am talking about is ...
... ('PILOT Data'!C341>=0,'PILOT Data'!C346>=0),'PILOT Data'!C341+'PILOT Data'!C346,IF('PILOT Data'!C341="",0+'PILOT Data'!C346,IF('PILOT Data'!C346="",0+'PILOT Data'!C341))))))))
It looks to me as though you are adding "na" to "n/a" (or something like that). Have you forgotten to make the cells B341 and B346 at this point?
I hope this is a good enough explanation of what I mean ...
Also, as far as I can remember you are only allowed 7 nested "IF" statements. Im not sure if it counts the first "IF" in that 7, but it might be worth looking at. I see 8 "IF"s in your expression.
Please let us know how you get on.
I really appreciate thank you messages as a payment for solving issues :o)