I always use the temperature as a gauge for me - my awareness and a general rule of thumb, but really make a decision for additional ventilation or heat based on their behavior. There have been times when I have adult guineas panting at 75 degrees and other times when they are fine at 90 degrees.
The same goes for keets. I always have an extra heat source at night and into the morning until the sun warms thing up, through their first month and continue it if the temperature inside the coop will be dipping into the 60's at night. It's funny, we're in New York and mine seem to do better with cooler temps, yet friends in the south and mid-west who have guineas find they tolerate the warmer temps better than mine do.
I use an infrared bulb that sits on top of one crate and a sweeter heater (infrared) that I raise and lower on another side. Usually into the 3rd week, they are shut off during the day and turned back on at night.
The coop / barn has a greenhouse fan installed in it and as the temperature warms during the day, I partially open a window and turn the fan on. I don't put a fan directly on them or let the air cause a draft on them. Mine are now 3 weeks and they seem to be fine in 75 degree temp - up, moving around, not huddling together and at 85 degrees, I turn on the fan to exhaust the hot air or if I see them panting.
Hope this helps with a guideline. Your situation may be a bit different, but it's always good to start with ideas from others and go from there. Good luck, enjoy and

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