you need a fan blowing on the plants all the time, especially if you grow in a closet......
the fan will:
a)help you to control the height of the plants
b)keep temp down
c)circulate air

it is therefore wiser to have a fan blowing on a 24/7 basis on the plants.....since you plan to do it in a closet thing with doors closed, it is going to get HOT in there no matter how far you put the lamp from the top of the plants.....especially in the case that there is no continuous airflow/exchange of air between outside and inside the closet/growroom.....don't worry about the power that a fan consumes...it is very low comparing to that a HID lamp consumes.....for my setup i use 4 computer fans that consume 3 watts each.....normal 3 speed oscilating fans consume up to 50 watts max on top speed......you do not need to use max speed though....just a wind like a breeze on your plants.... hope that helped