So over the years I have been getting better at increasing my smelt to fuel ratio.
When I set up a nice wind block pick warmer days and have everything ready I get right around 500lbs lead for a 15lb blue rhino tank.
Some times I do a little better but I feel like 30lbs of smelt to 1lb of fuel seems to be a good starting point.
I scored a used tank last week with 6lbs of fuel left in it. Previous owner said local spot would not fill it because it was a blue rhino tank. took it home and weighed it. 24lbs with a tar weight of 18lbs---smelted down 75lbs of range scrap and flashing used 1.8lbs fuel.
Definitely loose some efficiency when you get low on fuel the burner does not have the same umph. I will run the free tank to empty and go exchange it right now local exchange for blue rhino is 14.98