My approach is to set up the main light, trigger it via the flash meter and take an incident light reading at the subject position. Then add the fill light and meter for the fill. The fill should be less than the main light - how much less will depend on how contrasty you want your lighting. Then meter the background light to determine how the background will look - if you're going for high key, then the background should measure 2-3 stops over the main light, otherwise you will just want to know how bright or dim it is to effect separation between the main subject and the background.