FSC and furniture

That chair that you are purchasing would have to come from a FSC Chain of Custody certified manufacturer. The manufacturer of that chair will "label" or make a Claim about the FSC Content in that chair.
The burden of figuring out the FSC content of that chair does not fall on you the end user. It should be provided clearly on shippers and invoices from your FSC COC Manufacturer, or from the vendor that is selling the chair.

Do you have an example of a chair that you are using and trying to figure out the FSC content of? Send me a link and I'll point you in the right direction.

The acceptable forms of FSC Claims for LEED are FSC-XX%, FSC Mixed Credit, or FSC Pure.
If the wood chair is sold as FSC 90% than you would calculate 90% of the cost of that wood chair as FSC and 10% as Non FSC.
For either FSC Mixed Credit or FSC Pure, you would count the entire cost of that wood chair as FSC.