Old post but figured rule of thumb would be helpful for others. 10 amps per .010 thickness. So 1/8 or .125 125 Amps. 1/4 or .25 250 Amps. Now obviously this is for single pass full penetration and you can + or - a little so I'd say 200 Amps will do for 1/4.
Welder type plays into it as well. Mig tig arc. In tig you can get away with a lot and often I have the tig maxxed out and control the rest with the pedal. Tig is certainly not the ideal welder for building smokers though.