With basic setups you can get by with trial and error, but still it's easier to use the modeling lights. I keep my modeling lights set to track the power setting, so working in a darkened studio is essential for me to be able to see. I've developed a feel for the lights and modifiers, so I can get pretty close, then flip on the lights and fine tune. You'll be surprised how much easier it is to see the light in real time, especially if you kill the ambient. I've never been happy with zone focus on studio shots, I'm to anal about eyes. Using the select multi focus on tripod, 50mm, f7.1 and 15 ft away kept enough DOF to get the whole set in focus. Select multi focus on the subject assured me good sharpness on the eyes. If your camera supports it facial tracking isn't a bad idea on little ones, where you're working with a more limited DOF.