We used one gallon..it covers something like 500 sq ft.
I wouldn't try the 850 series on a single wall firebox. It's rated at 850 but it doesnt make it that far. You'll probably get some flake on the box.
* XL [COLOR=seagreen]BGE[/COLOR] *Charbroil offset (cheapo) and a *[URL="http://www.bbq-brethren.com/forum/showthread.php?t=125966"]project in the making 6' long x 28" diameter offset[/URL] [COLOR=red]<<<[/COLOR]Take a look!