Write what the room must hold before anyone draws it. For white button, ICAR-DMR's species table gives a spawn run at 22 to 25 °C and fruiting at 14 to 18 °C. NHB's model DPR warns that temperatures above 26 to 27 °C during spawn run and after casing should be avoided, and cropping kept below 18 °C.
That is your design brief. In a plains summer it means removing a lot of heat every hour. In the mid hills, almost none.