That's the million dollar question. Or should it be the 20 dollar question? I used a very scientific method when I built mine. It's styrofoam, w/thermostat/fan/2 x 40W bulbs, about 14 x 15 x 8". After I got it wired and running, I poked a 1/2" hole in the top, and an other one by the fan intake. Then let it run for a while. Where ever I found a warm spot, I took a sharp pencil, and made an other hole. I kept making holes here and there. Some for air ventilation, some because I wanted to drop a thermometer sensor in a particular spot. If I didn't like a hole, I covered it with duct tape. The best suggestion I can give you is to find some one who has a bator similar size to yours, and go by the hole set up they have, based on location of heating element and fan. Not having that as an option, just start making holes. It would make sense to make a hole for drawing in fresh air, and one at the top for venting out extra warm air. Otherwise, as long as you can hold temp and humidity, IMO you can't have too much ventilation. Those little egglets need lots of air.