That depends. You want it thicker for brushing, thinner for spraying. For all I know, if you've got an HVLP system you'll want a different viscosity than for old-style.
Now, if that doesn't make things vague enough, different brands of dope are thinner or thicker in the can -- Sig and (I think) Brodak are thinner, Randolph's is thicker.
Ideally you thin enough to get the right consistency, not to get some specific paint/thinner ratio.