How should the pack flow knob be set on an Airbus A320? On the Airbus A320 there is a pack flow knob and you can select LO(80%), NORM (100%), or HI(120%) according to the number of passengers and ambient conditions. Someone told me you select LO if there is less than 115 passengers HI if it is abnormally hot and NORM if everything is casual.

Do both numbers of passengers and ambient conditions have to be met to select LO,HI, or NORM? 

For example, obviously if it is above 100 degrees Fahrenheit you would select HI, but what if it is in the 90s or high 80s? Is there a specific temperature or is it option based?
No. Assuming there are no regulations, you can set it anyway you want, at an expense of fuel consumption. 

Do not confuse Ventilation control with Temperature control. Temperature control is another setting. (via Cockpit overhead/FAP)