Eleanor Kellon, VMD
Requirements are both by weight and by what it takes to keep minerals in balance.
Not sure where you came up with that 3.10% number (? multiplied by pounds?) but that's not correct. % is a description, like color. If a jar of jelly beans are all purple, one of them is purple too. If a gallon of chocolate milk has 5% chocolate syrup, a glass of it has 5% chocolate syrup too. Your hay is 0.28% magnesium whether it's 5 lbs or 50 lbs.
You have to change pounds to kg, change kg to grams then multiply by percent as a decimal. She is getting 12.7 g from the hay and 1.4 g from the cubes.
Eleanor in PA