How does the plasma cut manually? If you're able to get good cuts by hand you can measure distances and estimate speeds to get close to a starting set of figures to work from.
I have a plasma cutter that's currently manual but has remote switching for automation, the torch it came with has cut 1mm through to 10mm neatly without the need for nozzle swaps, are you delivering enough air pressure at the machine while it's cutting to provide sufficient flow? Low air delivery will give you wide, wandering and messy cuts,

- Nick