Tipping on a touch up

I'm going in tomorrow to have some minor touchups done to a tattoo and wondering how to tip. Of course I tipped for the original job (and I think I was generous. There's so much conflicting info out there, but I feel like I was fair) but what do I give for the touchup? It probably won't amount to more than 30 min work and the shop charges $150 an hour. I plan to return to this artist for at least 3 more largish pieces, and my husband is scheduled to have a big piece done next month, so we want to maintain a good relationship with this guy!

In my experience people usually tip $10-$25 for really fast touchups. If it takes longer than 30 minutes adjust accordingly. Try to pay for the supplies and part of lunch.