Usually 2% is the starting point. 60% meats and 40% veggies, or thereabouts. Then like any feed, adjusting to the individual dog will dictate the amounts per meal.

40lbs X 16ounce/lb = 640 ounces
640 X .02 (2%) = 12.8 ounces per meal
7.68 ounces of meats
5.12 ounces of veggies

Then adjust as necessary to the dog. EWO