6" or 8" to be safe to start, but using it vertically is a bad idea since most of the light comes off horizontally and just mounting vertically alone cuts HID bulbs output even further.
Hardly any light comes off the end of the bulb. Compare holding your hand under the end and then off to the side, see how much brighter off to the side is...
Also don't trap that heat in with foil, it's just for cooking turkeys and stuff.