I mix about 30% scratch to my girls laying portion and throw a little of the mix out for them to peck at. The rest goes in the feeder. I keep the feeder filled so they eat when they need to.
I find it hard to adjust correct levels of protein, calcium and other chemicals desired for good laying hens if you mix in outside grains. Use scratch only as a treat, once or twice a day. I would not mix it into their laying feed.