A 2x4 with wide side up works reall good. Its flat for their feet. I have the ones in the coop about 20 inches over my nest boxes which have I poop boards on. The roost I have in the run is about 3 1/2 ft off the ground.
Your silkies may want a very low perch or not use one at all, from what I've read. If the chickens get big and heavy enough, especially with a breed like orps, they may want it lower, or even want a shelf.
Chickens usually prefer to fly horizontally off a roost, unless they decide to walk down something like a ladder. How high depends partly on how much room they have to get down. My roosts are 30" and they fly about 6' out to get down. Then there's the one that roosts 6' off the floor, but she is young.
Some people use tree branches, but chicken feet don't wrap around a thin perch the same way most other birds' do. I think they do better with the 2x4 as mentioned, or maybe a nice fat tree branch, something like that.