I have used a similar scheme to find the nozzle offsets on my Quad machine. I have "spotting dye" here for use in the machine shop. It is similar to artist's Prussian blue oil paint, but washes off more easily. I just put a tiny smear of the dye on my finger and wipe on the nozzle. Then, press the nozzle down to the board and raise, and it leaves a round print like the letter "O". The dye wipes off easily from the nozzle.
The actual dye I use is made by Canode, but is not easy to get. The dye leaves a much more visible mark on shiny steel parts of the machine, but will work on a PC board, too, with less contrast.
The guy who taught me how to use the machine prefers to put "Silly Putty" (a children's toy item) on the board, and the nozzle will leave a round ring in the putty. But, then you will have to clear the nozzle for sure.
Jon