You don't really need much in this case. If it's a water insoluble ketone and you're extracting it from an aqueous phase, then you'll obviously be able to separate some of the ketone without a solvent, then just extract the aqueous 3x with a small amount of solvent.
You're talking about using like 1ml organic / 5mL aqueous... the best way to find these amounts is by trial and error. Keep track of your yields!