It really depends on your predators and your ground, etc. lots of factors.
A lot of folks do 18" and it works for them. If you have a coop camera, you should be able to tell a lot about what's going on once you get your chickens in place.
We have pseudo woodland here, with a small stream...
Sound like you're on top of the gravel issue. I tried filling my coop with 4-6" of wood chips, which somehow went down to 0-2" wood chips (need to refill), and bare dirt, and the chickens dig 6" holes if I let them.
I love my apron too. I think I ordered 4 ft hardware cloth and bent part of...
Be advised - your chickens will dig up your gravel if they can get to it. Mine live in an enclosed open air coop-run year-round. I used gravel to level the ground underneath the 2"x4" frame that I attached the greenhouse frame to, then ran hardware cloth over the top. They have dug the...