Depending on how many grains your scratch has (mix it in in higher concentrations for example if it is 11 grain scratch with higher protein), it can be successfully mixed into feed if you start with a nice high protein percentage feed such as Flock Raiser (20%), with oyster shells on the side for the layers.
The simplest thing to do is to only buy layer pellets or crumble (pellets usually waste less), as they are usually 16% protein, and throw out a handful of scratch per day or two, depending on flock size. I have 58 right now, so treats to me means a horse scoop quantity, which I throw everywhere to give everyone a chance at it. I still recommend oyster shell being available in case they need it.
Corn has around 7-9% protein, and the scratch that I have bought in the past (which has wheat in it too) is 10% protein. So since eggs are protein, you can see that you will probably get a lot more eggs if you feed higher protein.
Free ranging can give them a lot of their diet, but I recommend giving them at least 16% protein when they come to the feeder. You can throw pellets on the ground too, but when the rain hits them they turn to mush and moldy eventually. I do throw limited quantities of pellets around when I have them (not often).
I like my mix (see my BYC page) because if they don't eat it all after I broadcast it everywhere, it sprouts and I get lots of millet plants or wheat grass growing (happening right now, lol) which they chow down on too.