Adding 0.01" to the nozzle diameter will get you close… BUT the only way to zero in on your actual kerf width is to do a test. I do a 2" square with a 1" round hole… I suppose you could do a 1" square with a 1/2" hole. Then set it to cut the center of the line. The difference between what you designed and what you ended up with is your kerf width. Keep in mind your kerf width will change (get wider) as your tip ages.
Also, your kerf width will change with material thickness and amps as well.
Maybe this will help you.
This is what I have found to work with most plasma cutter nozzles/tips that most people have in their shops or garages. NOTE: Orifice hole size 1mm = .03937"
20 amp tip - 0.6mm/.025" - 15 to 20 amps - 50 to 55 psi
30 amp tip - 0.8mm/.030" - 21 to 30 amps - 55 to 60 psi
40 amp tip - 0.9mm/.035" - 31 to 40 amps - 65 to 70 psi
50 amp tip - 1.0mm/.040" - 41 to 50 amps - 65 to 75 psi
60 amp tip - 1.1mm/.044" - 51 to 60 amps - 65 to 75 psi
70 amp tip - 1.2mm/.047" - 61 to 70 amps - 75 to 80 psi
80 amp tip - 1.3mm/.051" - 71 to 80 amps - 75 to 80 psi