by 300 watt do you mean 68 watt or an actual 300 watt CFL? i have never seen a CFL over 250 and those normally cast as much as a 600 watt hps.
If in fact it is a 68 watt, i use them for side lighting and i would say 6 inches is a reasonable distance is you have good 78 degree temps and a fan. if your temps are hotter or you don't have a fan i might back it off to 8 or 10.
If it is in fact a 300 watter then i have no idea, i would say 12 to 14 inches most likely.
as far as hanging i would stop by a hardware store and pick up a large sized ceramic reflector for it for about $12 and it will have a bracket you can clip to anything or removes and just let it hang by the cord.