Definitely toss this out the window. My 3 week olds were fine at 48 degrees without any supplemental heat, because I raised them outdoors in those types of temperatures. The "5 degrees a week" thing is from my understanding a generalized guideline for commercial use to maximize growth or minimize losses.This is what google said- The Formula holds that brooder temperatures should be kept between 90-95° F for the first week after hatching, and reduced by five degrees each week thereafter until the brooder temperature equals the ambient room temperature.
You temp should probably be closer to 55, and you can start to ween them off the heat quickly now. You said it was 30 degrees out?
Yes on ventilation. Adding openings provides both necessary air flow as well as light, so no light should be needed inside if you plan according to light and wind directions. Is this set up in a barn or other enclosure? If so, wind direction might not be much of an issue at all, so you can go generous with windows.