I am not entirely sure this is correct, but here what I think.
Take the kcals per cup and divide that by the kcal per kg. This information is eitehr on the bag or on their website. That will give you kg per cup. Multiply that by 1000 to give you grams per cup. Multiply that by the percentage to give you grams of protein per cup.
For example, Merrick chicken and brown rice has 440 kcal/cup and 3638 kcal/kg. 440/3638 equals .1209. That multiplied by 1000 equals 120.9 grams per cup. It has 30 percent protein, so 120.9 multiplied by .30 equals 36 grams of protein per cup.