Whatever your calcium supplement, it can't be mixed with feed or the hens won't have a choice whether they eat it or not. It needs to be in a separate container.
Calcium and phosphorus are very closely related to each other and messing with that balance can have dire consequences. Chicken feed is formulated with the proper balance. Approximately 10:1 for layer feed, 2:1 for all other chickens. Deficiency or excess of one can interfere with the proper utilization of the other.
By adding a calcium source to the properly formulated feed, the disrupted balance of these minerals can result in pathological conditions including renal failure and development of gout.
Kidney damage always remains hidden until 24 hours before death with no symptoms. A hen can remain productive if there are still 2 of the 6 sections of kidney still functioning.
I keep a pint container of oyster shell by the water or feed with each flock. I sometimes add another container near the nest boxes. I usually have 2 cage cups side by side - one for oyster shell, one for grit and hang them on the side of the run wherever there is rain protection.
I use something like this.
http://www.ebay.com/itm/12-pcs-Gray...005&rk=2&rkt=6&mehot=pp&sd=310895602632&rt=nc