HELP! - Poop Hammock or droppings board specs

I am putting in a poop hammock made of a bag of feed. I only have 3 hens (stop laughing), so I have 1 roost, 3 ft. long. My question is how far off the edge of the roost should the edge of the hammock extend, so that the poop falls on the hammock and I don't get poop falling to the coop floor? I've searched for guidelines, but can't find any.

My hens are Ameraucana, Barred Rock and Australorp. No bantams here.

Thanks for all your help!