Temperature is one of the most common complaints of guests. You will never be able to satisfy everyone in setting a comfortable room temperature. Too warm of an environment decreases concentration level and fosters day dreaming. The best way to gauge temperature is to know your attendees. Women and older adults typically prefer a warmer setting.
Men generally prefer a cooler setting.
If the group is mixed, consider setting the temperature in the cooler range and hanging some inexpensive shawls or sweaters on a coat rack in the back of the room. That way people who are cold can cover up without having to adjust the room temperature. The physical atmosphere can have a dramatic impact on your attendees. Houseplants are also effective and can be used many times. Smells are also important.