I haven't run a 520, but based on making steam with a 700 stoker and a 900 boiler, I'd vote for the number of teeth that fills your radiators with steam (without building up a lot of pressure). However, I wouldn't go any higher than the number of teeth for which you can put in enough air to get an efficient burn. I think there are other threads here that tell you what the pot should look like. If you're burning rice that has a lot of undersized pieces, the fire my not "breathe" very well, and you may need a high air setting relative to the feed rate to burn efficiently (and may not be able to burn efficiently at high feed rates). I know EFM no longer recommends burning buck in a 520 (cue for whistlenut on that) but to make steam I'd suggest using at least "big" rice.