I knew I could count on you to feed the creative process!Here's a couple of ideas:
Randall's mention of matte paint and chalk gave me another idea: if you are worried about getting a clean fluid line on the oddly-shaped tank, you could try using an actual fluid line. You could dust the entire tank with a fine powder like chalk or flour, and carefully dip it in a pan of water. That would leave a visible waterline that you could follow, and reproduce on the other side.
I like the single strip but am a bit apprehensive to do it as forum member Dragonclaw recently painted his bike with a single red stripe (albeit slightly different looking) and I wouldn't want to steal his thunder from that. I do like the red outline around the tank though........
But wow, that is genius!!! I love that idea because you'd get a perfect straight line no matter what the tank is shaped like. If i have some version of the stripes I will have to try that for sure!
Thanks for the great help!!