It can also depend on paint brand, equipment setup and personal preference.
For example, Vallejo Model Color paints are much thicker "out of the bottle" than say, Gunze Acrylics. Even within a single manufacturer's range, paint viscosity varies between colours, and can vary between production batches of a single colour.
Like Mike, I also use Tamiya acrylics, but I tend to use much more thinner - it suits the way I paint.
So take thinning ratios as a "general" starting point, but don't be afraid to vary.