I don't like the "per tank" notion because it doesn't take into consideration additional costs such as nitrox nor the extra services valet ops offer. (Yes, part of that is factored into the dive cost, I realize, but would you tip a high end salon that gives you scalp massages, wine and other benefits the same $ amount as one that just cut your hair and sent you on your way because both gave you hair cuts?) So I subtract taxes from the total (since I don't want to tip the government) and then do 15-20% of my total remaining bill.
Because I am there for so long and dive regularly, and because crews DON'T rotate continually with the guy I dive with, I tip at the end of my trip. That means they get a big chunk of change all at once. Since I have dived with them for so many years, service doesn't suffer one iota as they know it will come, eventually.
Almost always the owner lets me know if there will be a crew alteration the day before I dive, so that I can individually tip the "fill-in" capitan or DM on the day. And then I deduct that from my total tip. That may not work for everyone but seems to work for me. Yes, I keep a log of each dive day so I can figure all this out at the end of my winter stay.