Amount does not matter - look at what the dog's physical condition is telling you. Do you easily feel the last few ribs? Do you see a tucked up belly and defined waist that caves in? Do you feel firmness when you touch the dog or is it loose with many folds of skin? Do they have good energy and activity?
If dog is too heavy, reduce by a half cup until you find the right amount. If too light, add half cup until it is at correct size.
People who feed higher quality kibble with more calories per serving are likely to feed less than someone feeding a less calorie dense food. What my dog eats will not be perfect for your dog - you must find your dog's correct portion size and continue to fine tune as the dog gets older.
If dog is a picky eater, you can add a spoon of canned food and soak in water for 5 mins. Usually gets them to eat. If the dog is chronically a picky eater, I let them fast a couple days if they want to drag it out - they will all eat sooner or later if they are in good condition.