Since there seems to be no "rule of thumb" no pun intended
and I do have 210 amp 230 volt MIG I might as well pick up a roll of .035 and see how it runs. So far, my .030 tests look good up to eight inch while the 5/16" had a lot of weld bead fully wetted out on top but maybe only 50% penetration. I got good results on the 8th by turning the wire feed down from the recommended setting and welding slower. In fact, I started burning holes through the base material....so...I'm just now finding this machines sweet spot. I will grab a spool of .035 and see where that takes me.