Sorry about your girl, and it isnt your fault, it happens. This time of year is the worst. The sky gets all cloudy where I live, and you cant even see if a Coppers Hawk is flying above. The chickens have no idea if they are safe. Also, there are a bunch of young hawks that mommy is no longer feeding, so they are looking for something simple to snack on. What concerns me is that you may not have a baby hawk on your hand, as he took a large white rock? Young hawks in my experience only went after babies, and bantams.
Also, from what I hear from others shelter as in a run, doesnt really help. I have heard people say their chicken ran into an enclosing, hawk fallowed chicken, dead chicken. And if it was a gray day, your hen may have never seen him coming.
You mentioned your roosters, and I love my big guys, but I am going to suggest getting a few Gamefowl Roosters. They are much smarter, hardier, and better fighters. Their bright colors, make them stand out more as well. I have had some hawks take glances at my Junglefowl rooster, who was staring them down, and fly away.
My Junglefowl also tends to notice things more quickly then the other boys. He lets out a cry that sends all birdies into the bushes. Then he stands out in the middle of the yard, and watches the sky until the hawks are gone. The other boys also stand out, but all the hens wait till he gives a signal to come out. I have hawks come almost every day, and see him at work, and I havent lost any birds this year to hawks.
Also when I see a hawk I also run outside and spread my arms out, cry in a loud hawk cry to the bird, and eye him down, as a challenge. It makes me look crazy, but it has always got them to leave.
And try to get some crows if you can. Crows hate hawks. Sorry about your birds, Im sure you, and the chicks, will miss them lots.