Compare two columns as a percentage

Giday all
I have a real simple one that has kept me stumped for several hours
Basically, I have several inspectors inspecting vehicles, and i am trying to express thier individual pass fail rate as a percentage.
If the inspector fail more than they pass they have a negative pass rate, if they pass more than they fail they have a positive pass rate.
The issue I have is working accross zero, ie: 1 pass and 1 fail is 50%, but take one from the other and you have zero, and a div by zero error.
Massivly frustrating.
I have attached the sheet and I hope someone can help out
Thanks Darren

Thanks Roger, I didnt even think of multiple if statement, works exactly as I need. Thanks again