Thank you, you gave me some food for thought about how to approach the problem. I deleted the posts, because i clearly misunderstood your first reply - My mind was in a different aras of the problem when i read it.
I understand your method now, and it was somehow what i meant when i asked the forum the first time - If there is any "PID" controller for PO's.
Maybe someone did the calculations before and have an amazing way to do this, already coded in a rutine.
In my case i have a peltier dehumidifier (on/off controlled), a cool mist humidifier for the humidity side (also on/off controller).
I have a fridge with a normal compressor and a heating lamp for the very few times i would need heating (maybe never).
I was searching for a good place ot start in a premade rutine, that i could then get to know and slowly adjust as my wisdom got larger - Else i will start doing as you write above, but i will be ugly and rough code haha as i am new in both arduino and temperature/humidity control! 