If you are free ranging, they are getting most of their calcium needs from the greens they are eating. If in a coop and run, you can always offer oyster shell free choice if you don't feel your calcium levels are optimal. I wouldn't add them directly to the feed, though I have done this in the past to try an experiment, I found no difference in shell tensile strength by actually feeding OS in the feeds, but more likely put a strain on kidneys by forcing them to consume excess calcium.
I would mix that mash with the whole grains at a 50%, as you describe and this will benefit your chickens in the long run. Come spring and peak laying season, you can mix it 2 parts layer, 1 part whole grain for a slightly more rich feed to support their increased production.