Without knowing what they are free ranging on, the only responsible answer is either feed them ad libitum/free choice a commercially complete feed, and allow them to self regulate their feed intake (I recommend all flock/flock layer type feed with grit and oyster shell available, each in seperate dishes), or to do as I do and cull pretty constantly so you can get your hands inside your birds and better discern whether their dietary needs are being met, then adjust the amount of feed you offer accordingly (which encourages them to spend more time ranging, and less time at the feeder).
Mealworms? Just don't. They are entertaining to you, the chickens love them like we love candy bars in the check out aisle, but even a small qualtity of mealworms, routinely, can easily imbalance a chicken's diet.