lightmeter is a must. one thing about muslin i hate is wrinkles. coloring black muslin with gel is tricky but it worths trying.
you have nice meter. it should work wired and wireless. and you can ask someone to hold meter and trigger lights yourself. i have old minolta meter and its very easy tool.
set iso 100 (camera and meter)
set sh/s at both lets say 1/125s
set both f/8
set front light and fire it somehow and read your meter (locate it carefully as in meters' manual. accordingly adjust power of front light. it'd show in gn or f/stops.
the same for back light and so on.
I just can't light the BG up with one 580EX II as my two main lights are used for key light and hair light. The wrinkles is frustrating too. I am thinking of buying a steamer. But If I can't light up the BG with color, there is no point to reveal the wrinkles or no wrinkle, right?