Humidity would play a significant role in the need to tunnel-ventilate young birds. The higher the humidity the more likely you would need to tunnel-ventilate a house. The guidelines provided in the article should provide a starting point for house operation. The best way to know if you need more air speed is to monitor the front to rear temperature differential as well as the birds. If the temperature rise becomes excessive or the the birds appear heat stressed fan capacity/air speed should be increased. Sorry to say there is no formula for optimal house operation.