The way I figure out how high to put the roost is look at the bedding on the floor. How high is that? Then position the nests. Then put the roosts so they are noticeably higher, usually about a foot higher works well.
If you use a ladder type roost, you generally need about 12” vertical and 12” horizontal separation so they are not pooping right on top of the ones lower down. The roost needs to be about 12” or more away from the wall to give them room.
I don’t have any experience with Silkies but they can’t fly. You need a ramp or a ladder for them to get up there. My regular large fowl birds have no problems jumping/flying up 5 feet and could probably go more if they wished.
The higher you go the more open space below they need to land. You don’t want them banging into nests, walls, or feeders on the way down. That’s why I suggest you make it as low as you reasonably can and still make it noticeably higher than anything you don’t want them sleeping on or in.
I suggest you give them as much roosting space as you reasonably can. They don’t take up much space when they are on the roost and settled down, but they need enough room to spread their wings and fly up. Also, it’s not that uncommon for one to be a bully and be pretty brutal to a chicken at the bottom of the pecking order while they are on the roosts. This is what causes a lot of chickens to wind up sleeping in the nests. Those chickens getting picked on look for a safe place to sleep and the nests are usually the highest safe place they can find. I don’t have a magic number for you. The more chickens you have the less space you need for each one. I’d not go less than 9” per chicken and prefer 12” per chicken, but see what’s reasonable with your coop and its layout. Also plan for future integration. The more space you have on the roosts or in the coop, the easier this goes. So be generous with space and give them room to get away from a bully if you have one.
You can see how I attached my tree limbs so they are removable yet don’t roll or move. Drill a hole and put a large nail in. I find it handy to make them removable.
Good luck. Looks like you have a good start.