Light placement

I am doing my first indoor grow. I have a book but it failed to say how far from my plant my light should be. The plant (green martian indica) is about 6" tall. I have a 400 watt HPS. I know MH is better for vegg but I only have one bulb and not enough money to run both yet.
So I digress, How far from my plant can I put the light?
I do not have a Hood with glass but a cheap hood that just reflects so there is a lot of heat getting down to the plant.
The plant started to curl up like a taco so I lifted the light. IT is about 18" away now and it seems much less stressed.
Is the 18" too much and if so how can i cool the light down when I can't afford a nice vented hood?

you should be fine at about 12 inchs. running the same kind of light at that distance.

Thanks. I was afraid of losing some lumins because of it being too far away but I don't want to cook my plant either.

Having a small fan pointed at the bulb will help a little, Maybe let ya lower it a few more inches.

I seen a tube thing that has ducting that I think may fit over the bulb and allow me to now buy an expensive hood. I will be going to my local hydro store manana.