I give mine free choice oyster shell. They ran out for a while and got into the egg eating habit (hard to break!), but are doing better now that they have their shell back. Now I have to collect eggs more often for a while and hope to break the habit.
Generally, the best diet for layers is layer ration with oyster shell available free choice. Scratch grains should be a treat only. They will eat less of the balanced layer ration when scratch is available and that can throw off their protein intake. I'd save the scratch for an occasional treat and see if that helps.