I don't see this method working very well for me since I always use duvets that are wider than the bed, which I thought was the norm. Same width as the bed makes for tug-of-war with the sleeping partner.
120 cm wide bed -> 150 cm duvet,
140-160 cm bed -> 240 cm duvet
120 cm wide bed -> 150 cm duvet, 140-160 cm bed -> 240 cm duvet