For my grit and oyster shell, I mix them in this way (these are all approximations):
My feed storage container in my garage holds about 50 lbs of premixed seeds and grains.
I add 1/2 cup of oyster shell to 4 lb of feed per day. Quite often I skip a day. So average is 6 cups of oyster shell per 50 lb. feed.
I add 1/2 cup of grit size #3 for adults, and #2 for bantams (#1 is chick grit) to 4 lb of feed every 2-3 days. So average is maybe? 4 cups of grit per 50 lb feed.
I have no idea what the commerical recommendations are. My hens are happy and healthy with very nice thick eggshells. When they don't want to eat something, they leave it in the feeder.
Periodically (about every 2 weeks) I toss the "bottom of the barrel" picked-over feed out on the ground and let them pick over it for a few hours before I give them fresh food in the feeder. This includes pumpkin seed hulls, old oats that no one wanted, grit and oyster shell that wasn't consumed, and generally some flaxseed and Nyjer thistle seed they wouldn't eat in the quantities I wanted them to.
It's all free choice feeding. In these amounts though, I don't have too much waste at all.
