There is no magic way. Whatever you think is fair is the way to go.
One way:
Everyone Uses the Place. Julie has established this base cost.
Wendy 1/3
Carla 1/3
Madeline 1/3
Melinda 1/3
Angie 1/3
Kelley 1/3
Lynn 1/3
Julie 1/3
Bedroom Usage
Wendy 1
Carla 1/2
Madeline 1/2
Melinda 1/2
Angie 1/2
Kelley 1/2
Lynn 1/2
Julie 0
All Told:
Wendy 1 + 1/3 = 8/6
Carla 1/2 + 1/3 = 5/6
Madeline 1/2 + 1/3 = 5/6
Melinda 1/2 + 1/3 = 5/6
Angie 1/2 + 1/3 = 5/6
Kelley 1/2 + 1/3 = 5/6
Lynn 1/2 + 1/3 = 5/6
Julie 0 + 1/3 = 2/6
We don't really need the sixths.
8 + 5 + 5 + 5 + 5 + 5 + 5 + 2 = 40
Wendy 8/40 * 1600 = 320
Carla 5/40 * 1600 = 200
Madeline 5/40
Melinda 5/40
Angie 5/40
Kelley 5/40
Lynn 5/40
Julie 2/40 * 1600 = 80
Checking: 320 + 6(200) + 80 = 1600 - Check
Again, I MUST emphasize that something EVERYONE thinks is fair is the way to go. If Miss Julie is just being a martyr and sleeping on the couch, then why should she pay less? If no one can possibly get along with Wendy, maybe she should pay even more! Perhaps one or a few of the participants went to the personal trouble to make all the arrangements? Maybe they should pay less. Perhaps Angie has recently gone through a divorce and the others feel sorry for her. Maybe she should pay less. Perhaps Kelley is a convicted felon and this caused the landlord to raise the rental fee. Maybe Kelley should pay more. Madeline and Melinda had to travel the farthest. Maybe they should pay less.
You can get as complicated as you like. If no one thinks it's fair, then you're done. Advice: Start simple and complicate only as necessary. You still want to be friends after you all go home.