I suggest you read this article. I think it gives good guidelines.
Oregon State - Feeding Chickens
http://extension.oregonstate.edu/catalog/html/pnw/pnw477/#anchor1132074
Since you let your chickens roam 2 hours a day, your circumstances are different than the basis of the article. They are getting a lot of extra stuff that is not considered in that article, seeds, greens and vitamins as well as bugs. I do think you need to pay a lot of attention to the comments on scratch however. It's like feeding candy to a child. A little is OK but it is easy to give too much.
If they are roaming a couple of hours a day, they are getting all the grit they need. Don't worry about that.
When you switch from grower to layer, you might cut backa bit on the greens from the garden so they will eat a bit more layer. It has extra calcium and other stuff they need for egg laying. Again, since you let them out as you do, it is not that big a deal since they are getting quite a variety while they are out. They will get calcium from some of the stuff they eat while out, such as bugs. If you are in limestone country, they will get extra calcium from the grit they pick up. But watch your eggs once they start laying and let the chickens tell you if they need extra calcium. If the shells are thick enough, you are doing things right concerning calcium. If the egg shells are thin, you should offer oyster shell free choice.
Sounds like you are taking good care of your chickens. All this is not an exact science for us backyard hobbyists. If we were running a commercial operation with 10,000 laying chickens in each of 5 chicken houses where they never get out to roam and chase bugs, it would be different. But our chickens can thrive in a wide variety of conditions if given a chance.