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Using max gross takeoff weight is the usual. If you're calculating something which is relevant to a specific scenario, using actual or expected operating weight is apppropriate (for example, wing loading in the landing configuration in transport-category aircraft is an interesting number and will be based on max landing weight, which is often way below max takeoff weight), but if the numbers are being tossed around in isolation they'll be based on max gross weight (at takeoff if applicable).