Optimum ratio of line width to layer thickness for overhang performance? When I have overhangs in my model, Cura colors them red. However, I noticed if I make layer thickness thinner, the red area is reduced or disappears. 

This could mean that thinner layer thickness is better for overhang, but it could also mean that a larger ratio of line width to layer thickness is better. It makes sense that if line width is 4X the layer thickness (such as 0.15 layers with 0.6 line width), overhang performance should be better than if line width is only 2X (such as 0.3 layers with the same 0.6 line width.

Is there a model that explains the optimum ratio of line thickness to layer height? Is only the ratio important, or is layer height also important by itself?
Cura high-lights overhangs in red if the printer would end up printing completely into thin air. 

If your overhang is angled instead of being a purely vertical-to-horizontal transition, a much thinner layer height can increase the chances that enough of the prior layer exists to support the next layer being printed. 

Basically, the thinner the layer (within reason), the greater the allowable angle of overhang that is safe to print.