Hi and welcome to the site.... we love pictures so be sure to post some.
Food --- Royal Canin is not a very good food, you may want to review the list of food ratings on the nutrition thread. As for how much... no more than 3 cups per day and break it up into 2 or 3 feedings. After a year, drop it to two cups a day.
Im not familiar with that food as far as quantity goes. The better the food, usually the less you need to feed because they utilize what is given to them better than a food that has fillers and such. Princess Gracie gets 1.5 cups a day broken up into 3/4 cups both morning and night. She is on Fromm Beef Frittata. When you have to feed less, the 'expensive' food seems less expensive.