What is a good amount of glucosamine?

Right now we feed our one dog which is a King Shepherd, Royal Canin GSD food which has 900mg glucosamine in it along with large breed Science Diet. We were thinking about switching his food to Blue Buffalo which only has 400mg of glucosamine in it. I'm concerned with my dogs joints because of his breed and because he is so large. Does anyone know if 400mg would be enough? I've heard Blue Buffalo food is amazing so I want to make the switch without possibly harming his joints in he process.