I'm trying to face my surface with a new (huge) cutter, and it keeps overshooting my soft limits on the machine. When I select an inch or 2 of Stock Offset it's not symmetrical. Then I noticed that it's never symmetrical! Reducing the size of the design has sort of worked... I would like to understand this behavior in Fusion.
Not quite sure what you mean. But Fusion will drive the edge of the cutter to the overlap. Sold let's say you have a 3" fly cutter and tell it to cut one inch past the stock. It will drive the cutter on inch past so there will still be 2 inches on the part making the toolpath appear to be non symmetrical even though it is doing what you told it.