Tricky question, as chainlines are not always intended to be straight anymore, in the sense of the middle of the cassette lining up with the middle of the chainring set. Scope out some of the factory-correct mountain bikes we've got now... my 100% kosher Shimano setup on my commuter here lines the middle of the cassette up almost directly to the granny ring. And I'm a cross-chaining fiend, so you can imagine what my chain goes through on a 48 x 34 stoplight launch

In the days of square-taper dominance, however, I generally looked for the inner face of the granny ring to be approximately at the BB shell's face on a 68mm shell.