Here's an idea and something I use to mark my sweet spot on my rooting needle. I take a zip tie and snugly apply it around my rooting tool (making it so I can still turn it), I clip the extra off, then when I find my sweet spot I turn the zip tie (cut end) to show where my sweet spot is. I tried marking the tool but I found this works so much better and when I change needles I just turn my tieto the new sweet spot.
Your going to tighten the zip tie so it doesn't just spin around or hang your tool but not so tight that you can't turn it with your fingers (does that make sense?) Like when you tighten you ties for your doll, you don't want them to fall off, or not be able to turn them, but turn easily.
The sweet spot I'm referring too is the place on your needle(barb ect) that picks up the hair for rooting.