Hi Mike,
what version of aspire are you running? I have version 8 (not version 9 as thought) and it doesn’t support double-sided machining the same way software like Fusion360 does.
If you have the same version, the way around it is to create one model for each side of the chair back. i.e one model for the convex side, one model for the concave side, ensuring that you set your Zero’s appropriately.
I think the newer versions of aspire support double sided machining - if this is what you have I’m not much help with that - maybe vetric forums or youtube would be a good place to look