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You can do it a number of different ways - use one of the presets or you can use one of those grey/color cards and set it as grey in the shot during conversion or use the dropper to sey grey on an object in the scene you thought was grey. The problem with that is it depends on the temperature of the light hitting it. Ultimately it boils down to a subjective decision to match what you saw as closely as possible to what the image displays unless you want to do something creative with a color shift or something.