It doesn't hurt having extra calcium available for them, they'll either eat it or not. Make sure you're giving them left overs that contain protein as well!
You never know if they're finding those essential worms and grubs when they're foraging.
I made a mealworm farm, its incredibly easy to set up and all it takes is a few small pieces of carrots every 3 or 4 days (depending on how many worms are in your farm).
http://www.instructables.com/id/Mealworm-Farm/
This website will tell you pretty much everything you need to know for your farm.
I don't think I'd be wrong in saying mealworms are one of chickens most loved treats. I've managed to train my chickens using mealworms.
For calcium, I give my ladies extremely fine eggshells. I boiled them, thoroughly clean them, then using a pestle and mortar, crush them as fine as can be.
The eggshells are then left in a separate container so they can eat as they need.
An extra warning, be sure to finely crush the egg shell and remove the membrane from the inside of the shell so your chickens don't recognize it as their own eggs, otherwise you'll start raising egg eaters!!