General spotting tips:

Use a really bright light in the workroom. The more, the better.

I've always preferred a larger diameter brush because I can make a finer point with it.

Sable, of course.

Put a few drops of slightly diluted Spot-Tone in a dish and let it dry. Then, use a very slightly damp brush tip to pick up the barest amount of dye, and test on a scrap print. You should easily be able to take out the offending dust speck... it will disappear before you think you've actually touched the print surface !