So tonight it was back out to the garage for another try. After making a few lines and thinking about how I was making an awfully huge bead, I went back to fiddling with the knobs again and this time started cranking down the feed rate. I realized part of what I had been doing wrong last time: too much wire. While I had the feed on 4 or 5, I should have had it much lower, like 2 (or even 2 minus). Once I turned the feed rate, I was also able to turn the power down. Way down. Instead of running at 5.1, 4.1 was a breeze, and I could even do 3.3 if I pulled the gun (instead of pushing).
I got things under control to the point that I was finally able to make a nice thin line, and that means: lettering. Drawing initials or one's name on a piece of metal is a crucial skill for the backyard metal worker. Its also good practise for teaching kids, cause they'll work at drawing their name cleanly. My name is too long, and has too many difficult letters; but my kids' names are easy.
So now the basics are under control. I still need to practise a lot more, but I have plenty of left over scraps of 1/8" from mess-ups on the plasma cutter. All that's left is to find another used gas mixer on Ebay, so I can make my own C25 gas mix with my CO2 bottle and Argon bottle.