I see the 0.040" nozzle diamter posted form my first attempt on the fogbuster. My second unit used 0.049" and worked just as well.
Much later, my line got cuaght in swarf and pulled of right to the mixing chamber. It made a good spitter with no line and I finished the job that way.
Anyway, the 0.040" dimension came from the patent and looks to not be that critical. I'd urge just using a mig tip if you build your own. Make a spare while you're at it. I've jerked the end of it at least three times.