Well your current board is probably fine because its not pulling anything through but otherwise yes, the vacuum will spread out within the spoilboard and go past your intended area. The gaskets in the plenum will help but still assume at least 8-12" outwards from that will have some air leakage as it spreads out. I cover up my un used areas with coroplast or offcuts but if you are doing a regular job then I'd make a custom sheet that covers your unused area or cut and seal the spoilboard to match the job. The former is what you can do if you need the table for other things and the later if the machine just needs to do one thing well.