Building a new chicken run, any advice welcome.

Hi all
I've had my 2 girls a couple of months now & desperately want to get a few more. We have a coop with small run but the girls free roam in the garden all day. I have a patch of waste ground at the end of the garden which I want to turn into a fully enclosed run, I would say its approx 10ft x 9ft. They will still get to roam in the garden but not all the time as its a nightmare juggling time in the garden for dogs & chickens.
I made loads of mistakes when I originally bought my coop and have spent the same again modifying it against red mite, predators etc.
I was wondering if there is anything I positively have to do to get this right, a shed runs down one side with two fence panels creating another side, I was going to put chicken fence panels along the front with a gate to the side, the top will be enclosed with chicken wire.
Any advice would be gratefully appreciated, also how many chickens would comfortably fit in the space, I love my Black Rock so will be tempted to get more of them or at least hybrids the same size

Perfect Dominic, just the sort of advice I'm looking for, thank you so much.