Don't machine top of stock n- Meshcam

Hi All
What’s the secret to getting the “Don’t machine top of stock” function in Meshcam to work?
I am experimenting with routing out a guitar body. The stock is flat on top and I just want the router to router the sides and the cavities out. However, every time I calculate the tool paths with the above function checked it still shows the entire top being routed out.
Any and all help is greatly appreciated.

You might check this out: Highline Guitars

That being said, I found Meshcam extremely primative and basically a one-trick-pony.

This is coming from someone with little CAM experience so take it with a grain of salt.