Please don't take this the wrong way but wainrights isn't a very good food it is full of fillers which are of no use to the dog so he might end up pooing more than normal 
can I ask why are you changing his food 
I think you might be thinking of a different brand. I don't feed Wainwrights but it's not a bad food, on a par with JWB ingredient wise. I would feed it before I'd feed Royal Canin Medium Adult to my dogs.
Wainwrights Adult Duck and Rice ingredients:
Brown Rice (38%), Duck (30%) (Duck Meal Meal, Duck Gravy), Barley (14%), Sugar Beet Pulp (8%), Rapeseed Oil (4%), Whole Linseed (3%), Alfalfa (1%), Minerals (includes Yucca Extract 200mg/kg, Marigold Meal 50mg/kg, Rosemary Extract 5mg/kg), Seaweed (0.5%).
Royal Canin Medium Adult ingredients:
Corn, poultry protein (dried), corn bran, wheat middlings, meat dried (beef, pork)*, animal fats, wheat, animal protein (hydrolysed), beet pulp, fish oil, minerals, yeasts, soya oil, yeast hydrolyzate (source of mannan-oligosaccharides).
To the OP as a general rule the better a food the less you have to feed of it as the dog gets more out of what's given. The quantities on the bags are only guidelines though, they're not set in stone. Adding vegetables is also a good way of bulking out food and making them feel like they're getting more.
