What do you feed your flock?

Okay, so at night, sometimes I focus my thoughts on how I can better my animals' lives, diets, activity, etc. Last night, I asked myself about the diet of my chickens. Are they getting enough to eat? Am I feeding them the right kind of feed? And ultimately came to the question that I would like feedback on.. Is layer's mash the right kind of feed to give to the entire flock? If it is, then great! If not, then, what should I give them in addition? Because that is what I have been giving them the past 3 years. They all seem to be doing well on just on layer's mash, but sometimes I wonder if they get bored of the same ol' meal every day. This flock does free range during the day, so they are able to find little goodies in their travels. They also get dinner scraps and their own egg shells back for calcium benefits. Do any of you mix the layer's mash with anything else?
Also, as far as layer's mash, what form do you give it in? I have been experimenting with the different varieties: pellets, crumbles, etc. This last bag of feed I purchased from a local ag. center and it's more like a coarse powder (finer than crumbles). I personally, don't like it, but I'm not the one that has to eat it. The chickens seem to approve, as it has a nice sweet aroma to it and has bits of corn and other "stuff" to make it a balanced diet. I went and threw away the ingredient list, so I can't tell you exactly what other "stuff" is in it. But when I read the label, it appeared legit to me at the time.
I guess the main reason I have these questions is because I want to be sure that my girls, and boys are the right weight. I worry sometimes that they might be too skinny- and I would rather have chickens that are a little on the chubby side than on the skinny side. Thanks for listening to me ramble on!