Hello!
A few questions: Do you hens get to free range at all? Do they have a chicken run and is their sand in it? I ask that because of your question about the grit. If you have sand in your chicken run, or if you allow them to free range, then you won't need to supply the grit.
As far as the feed goes, it's best if you keep your feeders filled at all times, so they can eat as much as they like. For 20 hens, I would keep three feeders filled and hanging. If you hang them, there is less waste, or set them up on something, like a crate, at their head/beak level, so they won't be able to scratch into it and waste the feed.
I keep feed indoors, and water outdoors too, just so you know. Keeping food outside is a no-no, IMHO, because it does manage to get on the ground, spoil, and if the chickens eat that, they can become ill.
I wouldn't feed alot of scratch. It's tastier, and they will like it much better than the layer feed, and that's where they'll get their nutrition for egg laying. You don't need to, but supplying some crushed oyster shell is a great idea, promoting strong egg shells.
Hope this helps you! Have a wonderful day,
Sharon